Method for forward progress and programmable timeouts of tree traversal mechanisms in hardware

ABSTRACT

In a ray tracer, to prevent any long-running query from hanging the graphics processing unit, a traversal coprocessor provides a preemption mechanism that will allow rays to stop processing or time out early. The example non-limiting implementations described herein provide such a preemption mechanism, including a forward progress guarantee, and additional programmable timeout options that can be time or cycle based. Those programmable options provide a means for quality of service timing guarantees for applications such as virtual reality (VR) that have strict timing requirements.

CROSS-REFERENCE TO RELATED PATENTS AND APPLICATIONS

This application is a continuation of Ser. No. 16/101,232, filed Aug.10, 2018, which is hereby incorporated by reference and is related tothe following commonly-assigned US patents and patent applications, theentire contents of each of which are incorporated by reference: U.S.application Ser. No. 14/563,872 titled “Short Stack Traversal of TreeData Structures” filed Dec. 8, 2014; U.S. Pat. No. 9,582,607 titled“Block-Based Bounding Volume Hierarchy”; U.S. Pat. No. 9,552,664 titled“Relative Encoding For A Block-Based Bounding Volume Hierarchy” as; U.S.Pat. No. 9,569,559 titled “Beam Tracing” filed Mar. 18, 2015; U.S. Pat.No. 10,025,879 titled “Tree Data Structures Based on a Plurality ofLocal Coordinate Systems”; US application Ser. No. 14/737,343 titled“Block-Based Lossless Compression of Geometric Data” filed Jun. 11,2015; and the following US Applications filed concurrently herewith:

-   US patent application Ser. No. 16/101,066 titled “Method for    Continued Bounding Volume Hierarchy Traversal on Intersection    without Shader Intervention”;-   US patent application Ser. No. 16/101,109 titled “Method for    Efficient Grouping of Cache Requests for Datapath Scheduling”;-   US patent application Ser. No. 16/101,247 title “A Robust, Efficient    Multiprocessor-Coprocessor Interface”;-   US patent Applicaiton Ser. No. 16/101,180 titled “Query-Specific    Behavioral Modification of Tree Traversal”;-   US patent application Ser. No. 16/101,148 title “Conservative    Watertight Ray Triangle Intersection”; and-   US patent application Ser. No. 16/101,196 titled “Method or Handling    Out-of-Order Opaque and Alpha Ray/Primitive Intersections”.

FIELD

The present technology relates to computer graphics, and moreparticularly to ray tracers. More particularly, the technology relatesto hardware acceleration of computer graphics processing including butnot limited to ray tracing. Still more particularly, the examplenon-limiting technology herein relates to a hardware-based traversalcoprocessor that efficiently traverses an acceleration data structuree.g., for real time ray tracing. More particularly, example non-limitingimplementations relate to mechanisms that enable a processor tointerrupt a coprocessor from processing rays for reasons such aspreemption or programmable timeouts, and to a preemption controlmechanism that ensures a coprocessor makes guaranteed forward progressbefore ceasing execution in response to a preemption request.

BACKGROUND & SUMMARY

If you look around the visual scene before you, you will notice thatsome of the most interesting visual effects you see are produced bylight rays interacting with surfaces. This is because light is the onlything we see. We don't see objects—we see the light that is reflected orrefracted by the objects. Most of the objects we can see reflect light(the color of an object is determined by which parts of light the objectreflects and which parts it absorbs). Shiny surfaces such as metallicsurfaces, glossy surfaces, ceramics, the surfaces of liquids and avariety of others (even the corneas of the human eyes) act as mirrorsthat specularly reflect light. For example, a shiny metal surface willreflect light at the same angle as it hit the surface. An object canalso cast shadows by preventing light from reaching other surfaces thatare behind the object relative to a light source. If you look around,you will notice that the number and kinds of reflections and the number,kinds and lengths of shadows depend on many factors including the numberand type of lights in the scene. A single point light such as a singlefaraway light bulb will produce single reflections and hard shadows.Area light sources such as windows or light panels produce differentkinds of reflection highlights and softer shadows. Multiple lights willtypically produce multiple reflections and more complex shadows (forexample, three separated point light sources will produce three shadowswhich may overlap depending on the positions of the lights relative toan object).

If you move your head as you survey the scene, you will notice that thereflections change in position and shape (the shadows do the same). Bychanging your viewpoint, you are changing the various angles of thelight rays your eyes detect. This occurs instantaneously—you move yourhead and the visual scene changes immediately.

The simple act of drinking a cup of tea is a complex visual experience.The various shiny surfaces of the glossy ceramic cup on the table beforeyou reflect each light in the room, and the cup casts a shadow for eachlight. The moving surface of the tea in the cup is itself reflective.You can see small reflected images of the lights on the tea's surface,and even smaller reflections on the part of the tea's surface where theliquid curves up to meet the walls of the cup. The cup walls also castshadows onto the surface of the liquid in the cup. Lifting the cup toyour mouth causes these reflections and shadows to shift and shimmer asyour viewpoint changes and as the surface of the liquid is agitated bymovement.

We take these complexities of reflections and shadows for granted. Ourbrains are adept at decoding the positions, sizes and shapes of shadowsand reflections and using them as visual cues. This is in part how wediscern the position of objects relative to one another, how wedistinguish one object from another and how we learn what objects aremade of. Different object surfaces reflect differently. Specular (mirrortype) reflection of hard metal creates images of reflected objects,while diffuse reflection off of rough surfaces is responsible for colorand lights up objects in a softer way. Shadows can be soft and diffuseor hard and distinct depending on the type of lighting, and the lengthsand directions of the shadows will depend on the angle of the light raysrelative to the object and our eyes.

Beginning artists typically don't try to show reflection or shadows.They tend to draw flat scenes that have no shadows and no reflections orhighlights. The same was true with computer graphics of the past.

Real time computer graphics have advanced tremendously over the last 30years. With the development in the 1980's of powerful graphicsprocessing units (GPUs) providing 3D hardware graphics pipelines, itbecame possible to produce 3D graphical displays based on texture-mappedpolygon primitives in real time response to user input. Such real timegraphics processors were built upon a technology called scan conversionrasterization, which is a means of determining visibility from a singlepoint or perspective. Using this approach, three-dimensional objects aremodelled from surfaces constructed of geometric primitives, typicallypolygons such as triangles. The scan conversion process establishes andprojects primitive polygon vertices onto a view plane and fills in thepoints inside the edges of the primitives. See e.g., Foley, Van Dam,Hughes et al, Computer Graphics: Principles and Practice (2d Ed.Addison-Wesley 1995 & 3d Ed. Addison-Wesley 2014).

Hardware has long been used to determine how each polygon surface shouldbe shaded and texture-mapped and to rasterize the shaded, texture-mappedpolygon surfaces for display. Typical three-dimensional scenes are oftenconstructed from millions of polygons. Fast modern GPU hardware canefficiently process many millions of graphics primitives for eachdisplay frame (every 1/30^(th) or 1/60^(th) of a second) in real timeresponse to user input. The resulting graphical displays have been usedin a variety of real time graphical user interfaces including but notlimited to augmented reality, virtual reality, video games and medicalimaging. But traditionally, such interactive graphics hardware has notbeen able to accurately model and portray reflections and shadows.

Some have built other technologies onto this basic scan conversionrasterization approach to allow real time graphics systems to accomplisha certain amount of realism in rendering shadows and reflections. Forexample, texture mapping has sometimes been used to simulate reflectionsand shadows in a 3D scene. One way this is commonly done is totransform, project and rasterize objects from different perspectives,write the rasterized results into texture maps, and sample the texturemaps to provide reflection mapping, environment mapping and shadowing.While these techniques have proven to be useful and moderatelysuccessful, they do not work well in all situations. For example,so-called “environment mapping” may often require assuming theenvironment is infinitely distant from the object. In addition, anenvironment-mapped object may typically be unable to reflect itself. Seee.g.,http://developer.download.nvidia.com/CgTutorial/cg_tutorial_chapter07.html.These limitations result because conventional computer graphicshardware—while sufficiently fast for excellent polygon rendering—doesnot perform the light visualization needed for accurate and realisticreflections and shadows. Some have likened raster/texture approximationsof reflections and shadows as the visual equivalent of AM radio.

There is another graphics technology which does perform physicallyrealistic visibility determinations for reflection and shadowing. It iscalled “ray tracing”. Ray tracing was developed at the end of the 1960'sand was improved upon in the 1980's. See e.g., Apple, “Some Techniquesfor Shading Machine Renderings of Solids” (SJCC 1968) pp. 27-45;Whitted, “An Improved Illumination Model for Shaded Display” Pages343-349 Communications of the ACM Volume 23 Issue 6 (June 1980); andKajiya, “The Rendering Equation”, Computer Graphics (SIGGRAPH 1986Proceedings, Vol. 20, pp. 143-150). Since then, ray tracing has beenused in non-real time graphics applications such as design and filmmaking. Anyone who has seen “Finding Dory” (2016) or other Pixaranimated films has seen the result of the ray tracing approach tocomputer graphics—namely realistic shadows and reflections. See e.g.,Hery et al, “Towards Bidirectional Path Tracing at Pixar” (2016).

Ray tracing is a primitive used in a variety of rendering algorithmsincluding for example path tracing and Metropolis light transport. In anexample algorithm, ray tracing simulates the physics of light bymodeling light transport through the scene to compute all global effects(including for example reflections from shiny surfaces) using rayoptics. In such uses of ray tracing, an attempt may be made to traceeach of many hundreds or thousands of light rays as they travel throughthe three-dimensional scene from potentially multiple light sources tothe viewpoint. Often, such rays are traced relative to the eye throughthe scene and tested against a database of all geometry in the scene.The rays can be traced forward from lights to the eye, or backwards fromthe eye to the lights, or they can be traced to see if paths startingfrom the virtual camera and starting at the eye have a clear line ofsight. The testing determines either the nearest intersection (in orderto determine what is visible from the eye) or traces rays from thesurface of an object toward a light source to determine if there isanything intervening that would block the transmission of light to thatpoint in space. Because the rays are similar to the rays of light inreality, they make available a number of realistic effects that are notpossible using the raster based real time 3D graphics technology thathas been implemented over the last thirty years. Because eachilluminating ray from each light source within the scene is evaluated asit passes through each object in the scene, the resulting images canappear as if they were photographed in reality. Accordingly, these raytracing methods have long been used in professional graphicsapplications such as design and film, where they have come to dominateover raster-based rendering.

The main challenge with ray tracing has generally been speed. Raytracing requires the graphics system to compute and analyze, for eachframe, each of many millions of light rays impinging on (and potentiallyreflected by) each surface making up the scene. In the past, thisenormous amount of computation complexity was impossible to perform inreal time.

One reason modern GPU 3D graphics pipelines are so fast at renderingshaded, texture-mapped surfaces is that they use coherence efficiently.In conventional scan conversion, everything is assumed to be viewedthrough a common window in a common image plane and projected down to asingle vantage point. Each triangle or other primitive is sent throughthe graphics pipeline and covers some number of pixels. All relatedcomputations can be shared for all pixels rendered from that triangle.Rectangular tiles of pixels corresponding to coherent lines of sightpassing through the window may thus correspond to groups of threadsrunning in lock-step in the same streaming processor. All the pixelsfalling between the edges of the triangle are assumed to be the samematerial running the same shader and fetching adjacent groups of texelsfrom the same textures. In ray tracing, in contrast, rays may start orend at a common point (a light source, or a virtual camera lens) but asthey propagate through the scene and interact with different materials,they quickly diverge. For example, each ray performs a search to findthe closest object. Some caching and sharing of results can beperformed, but because each ray potentially can hit different objects,the kind of coherence that GPU's have traditionally taken advantage ofin connection with texture mapped, shaded triangles is not present(e.g., a common vantage point, window and image plane are not there forray tracing). This makes ray tracing much more computationallychallenging than other graphics approaches—and therefore much moredifficult to perform on an interactive basis.

Much research has been done on making the process of tracing rays moreefficient and timely. See e.g., Glassner, An Introduction to Ray Tracing(Academic Press Inc., 1989). Because each ray in ray tracing is, by itsnature, evaluated independently from the rest, ray tracing has beencalled “embarrassingly parallel.” See e.g., Akenine-Möller et al., RealTime Rendering at Section 9.8.2, page 412 (Third Ed. CRC Press 2008). Asdiscussed above, ray tracing involves effectively testing each rayagainst all objects and surfaces in the scene. An optimization called“acceleration data structure” and associated processes allows thegraphics system to use a “divide-and-conquer” approach across theacceleration data structure to establish what surfaces the ray hits andwhat surfaces the ray does not hit. Each ray traverses the accelerationdata structure in an individualistic way. This means that dedicatingmore processors to ray tracing gives a nearly linear performanceincrease. With increasing parallelism of graphics processing systems,some began envisioning the possibility that ray tracing could beperformed in real time. For example, work at Saarland University in themid-2000's produced an early special purpose hardware system forinteractive ray tracing that provided some degree of programmability forusing geometry, vertex and lighting shaders. See Woop et al., “RPU: AProgrammable Ray Processing Unit for Real Time Ray Tracing” (ACM 2005).As another example, Advanced Rendering Technology developed“RenderDrive” based on an array of AR250/350 rendering processorsderived from ARM1 and enhanced with custom pipelines for ray/triangleintersection and SIMD vector and texture math but with no fixed-functiontraversal logic. See e.g.,http://www.graphicshardware.org/previous/www_2001/presentations/Hot3D_Daniel_Hall.pdf

Then, in 2010, NVIDIA took advantage of the high degree of parallelismof NVIDIA GPUs and other highly parallel architectures to develop theOptiX™ ray tracing engine. See Parker et al., “OptiX: A General PurposeRay Tracing Engine” (ACM Transactions on Graphics, Vol. 29, No. 4,Article 66, July 2010). In addition to improvements in API's(application programming interfaces), one of the advances provided byOptiX™ was improving the acceleration data structures used for findingan intersection between a ray and the scene geometry. Such accelerationdata structures are usually spatial or object hierarchies used by theray tracing traversal algorithm to efficiently search for primitivesthat potentially intersect a given ray. OptiX™ provides a number ofdifferent acceleration structure types that the application can choosefrom. Each acceleration structure in the node graph can be a differenttype, allowing combinations of high-quality static structures withdynamically updated ones.

The OptiX™ programmable ray tracing pipeline provided significantadvances, but was still generally unable by itself to provide real timeinteractive response to user input on relatively inexpensive computingplatforms for complex 3D scenes. Since then, NVIDIA has been developinghardware acceleration capabilities for ray tracing. See e.g., U.S. Pat.Nos. 9,582,607; 9,569,559; US20160070820; and US20160070767.

Given the great potential of a truly interactive real time ray tracinggraphics processing system for rendering high quality images ofarbitrary complexity in response for example to user input, further workis possible and desirable.

Some Ray Processes can Take Too Long or May Need to be Interrupted

Ray tracing generally involves executing a ray intersection queryagainst a pre-built Acceleration Structure (AS), sometimes referred tomore specifically as a Bounding Volume Hierarchy (BVH). Depending on thebuild of the AS, the number of primitives in the scene and theorientation of a ray, the traversal can take anywhere from a few tohundreds to even thousands of cycles by specialized traversal hardware.Additionally, if a cycle or loop is inadvertently (or even intentionallyin the case of a bad actor) encoded into the BVH, it is possible for atraversal to become infinite. For example, it is possible for a BVH todefine a traversal that results in an “endless loop.”

To prevent any long-running query from hanging the GPU, the exampleimplementation Tree Traversal Unit (TTU) 700 provides a mechanism forpreemption that will allow rays to timeout early. The examplenon-limiting implementations described herein provide such a preemptionmechanism, including a forward progress guarantee, and additionalprogrammable timeout options that build upon that. Those programmableoptions provide a means for quality of service timing guarantees forapplications such as virtual reality (VR) that have strict timingrequirements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example non-limiting ray tracing graphics system.

FIG. 2A shows an example specular object.

FIG. 2B shows the example object within a bounding volume.

FIG. 2C shows an example volumetric subdividing of the FIG. 2B boundingvolume.

FIGS. 2D, 2E and 2F show example further levels of volumetricsubdivision of the bounding volume to create a bounding volume hierarchy(BVH).

FIG. 2G shows an example portion of the object comprised of primitivesurfaces, in this case triangles.

FIGS. 3A-3C show example simplified ray tracing tests to determinewhether the ray passes through a bounding volume containing geometry andwhether the ray intersects geometry.

FIG. 4 illustrates an example ray tracing flowchart.

FIGS. 5A-5C show example different ray-primitive intersection scenarios.

FIGS. 6A and 6B show an example of how texture mapping can impactray-primitive intersection results.

FIGS. 7A and 7B illustrate ray instance transforms.

FIG. 8A illustrates an example non-limiting bounding volume hierarchy(BVH).

FIG. 8B shows an example acceleration data structure in the form of agraph or tree.

FIG. 9 shows a simplified example non-limiting traversal co-processorcomprising a tree traversal unit (TTU).

FIG. 10A illustrates an example non-limiting ray tracing shadingpipeline flowchart.

FIGS. 10B & 10C illustrate more detailed ray tracing pipelines.

FIGS. 11A-11H together are a flip chart animation that illustrates arough analogy to the TTU's preemption mechanism with guaranteed forwardprogress.

FIG. 12 shows an example process the TTU performs in response toreceiving a preemption signal.

FIG. 12A shows an example flowchart of a forward progress test.

FIG. 13 is an example non-limiting flowchart of a programmable TTU raytimeout arrangement.

FIG. 13A is an example non-limiting flowchart of an alternativeprogrammable TTU ray timeout arrangement.

FIG. 14 illustrates an example flowchart for generating an image.

FIG. 15 illustrates an example parallel processing unit (PPU).

FIG. 16 illustrates an example memory partition unit.

FIG. 17 illustrates an example general processing cluster (GPC) withinthe parallel processing unit of FIG. 15.

FIG. 18 is a conceptual diagram of a graphics processing pipelineimplemented by the GPC of FIG. 17.

FIGS. 19 and 20 illustrate an example streaming multi-processor.

FIG. 21 is a conceptual diagram of a processing system implemented usingPPUs of FIG. 15.

FIG. 22 expands FIG. 21 to show additional interconnected devices.

DETAILED DESCRIPTION OF NON-LIMITING EMBODIMENTS

The technology herein provides hardware capabilities that accelerate raytracing to such an extent that it brings the power of ray tracing togames and other interactive real time computer graphics, initiallyenabling high effect quality in shadows and reflections and ultimatelyglobal illumination. In practice, this means accelerating ray tracing bya factor of up to an order of magnitude or more over what would bepossible in software on the same graphics rendering system.

In more detail, the example non-limiting technology provides dedicatedhardware to accelerate ray tracing. In non-limiting embodiments, ahardware co-processor (herein referred to as a “traversal coprocessor”or in some embodiments a “tree traversal unit” or “TTU”) acceleratescertain processes supporting interactive ray tracing includingray-bounding volume intersection tests, ray-primitive intersection testsand ray “instance” transforms.

In some non-limiting embodiments, the traversal co-processor performsqueries on an acceleration data structure for processes running onpotentially massively-parallel streaming multiprocessors (SMs). Thetraversal co-processor traverses the acceleration data structure todiscover information about how a given ray interacts with an object theacceleration data structure describes or represents. For ray tracing,the traversal coprocessors are callable as opposed to e.g., fixedfunction units that perform an operation once between logical pipelinestages running different types of threads (e.g., vertex threads andpixel threads).

In some non-limiting embodiments, the acceleration data structurecomprises a hierarchy of bounding volumes (bounding volume hierarchy orBVH) that recursively encapsulates smaller and smaller bounding volumesubdivisions. The largest volumetric bounding volume may be termed a“root node.” The smallest subdivisions of such hierarchy of boundingvolumes (“leaf nodes”) contain items. The items could be primitives(e.g., polygons such as triangles) that define surfaces of the object.Or, an item could be a sphere that contains a whole new level of theworld that exists as an item because it has not been added to the BVH(think of the collar charm on the cat from “Men in Black” whichcontained an entire miniature galaxy inside of it). If the itemcomprises primitives, the traversal co-processor tests rays against theprimitives to determine which object surfaces the rays intersect andwhich object surfaces are visible along the ray.

The traversal co-processor performs a test of each ray against a widerange of bounding volumes, and can cull any bounding volumes that don'tintersect with that ray. Starting at a root node that bounds everythingin the scene, the traversal co-processor tests each ray against smaller(potentially overlapping) child bounding volumes which in turn bound thedescendent branches of the BVH. The ray follows the child pointers forthe bounding volumes the ray hits to other nodes until the leaves orterminal nodes (volumes) of the BVH are reached. Once the traversalco-processor traverses the acceleration data structure to reach aterminal or “leaf” node that contains a geometric primitive, it performsan accelerated ray-primitive intersection test that determines whetherthe ray intersects that primitive (and thus the object surface thatprimitive defines). The ray-primitive test can provide additionalinformation about primitives the ray intersects that can be used todetermine the material properties of the surface required for shadingand visualization. Recursive traversal through the acceleration datastructure enables the traversal co-processor to discover all objectprimitives the ray intersects, or the closest (from the perspective ofthe viewpoint) primitive the ray intersects (which in some cases is theonly primitive that is visible from the viewpoint along the ray).

The traversal co-processor also accelerates the transform of each rayfrom world space into object space to obtain finer and finer boundingbox encapsulations of the primitives and reduce the duplication of thoseprimitives across the scene. Objects replicated many times in the sceneat different positions, orientations and scales can be represented inthe scene as instance nodes which associate a bounding box and leaf nodein the world space BVH with a transformation that can be applied to theworld-space ray to transform it into an object coordinate space, and apointer to an object-space BVH. This avoids replicating the object spaceBVH data multiple times in world space, saving memory and associatedmemory accesses. The instance transform increases efficiency bytransforming the ray into object space instead of requiring the geometryor the bounding volume hierarchy to be transformed into world (ray)space and is also compatible with additional, conventional rasterizationprocesses that graphics processing performs to visualize the primitives.

The presently disclosed non-limiting embodiments thus provide atraversal co-processor, a new subunit of one or a group of streamingmultiprocessor SMs of a 3D graphics processing pipeline. In order tounderstand where the traversal co-processor fits in the overall picture,it may be helpful to understand a few fundamentals of the algorithmemployed by most or all modern ray tracers. But it should be pointed outthat the technology herein provides a generic capability to determine,for a thread running in a GPU, what the nearest visible thing is from agiven point along a specified direction, or if anything lies between twopoints. A common use case for such capability will be in processes thatstart tracing rays from points that have already been rasterized ontriangles using conventional scan conversion techniques. The disclosedtechnology can but does not necessarily replace or substitute for scanconversion technology, and may often augment it and be used inconjunction with scan conversion techniques to enhance images withphotorealistic reflections, shadows and other effects.

Ray Tracing Techniques

Generally, ray tracing is a rendering method in which rays are used todetermine the visibility of various elements in the scene. Ray tracingcan be used to determine if anything is visible along a ray (forexample, testing for occluders between a shaded point on a geometricprimitive and a point on a light source) and can also be used toevaluate reflections (which may for example involve performing atraversal to determine the nearest visible surface along a line of sightso that software running on a streaming processor can evaluate amaterial shading function corresponding to what was hit—which in turncan launch one or more additional rays into the scene according to thematerial properties of the object that was intersected) to determine thelight returning along the ray back toward the eye. In classicalWhitted-style ray tracing, rays are shot from the viewpoint through thepixel grid into the scene, but other path traversals are possible.Typically, for each ray, the closest object is found. This intersectionpoint can then be determined to be illuminated or in shadow by shootinga ray from it to each light source in the scene and finding if anyobjects are in between. Opaque objects block the light, whereastransparent objects attenuate it. Other rays can be spawned from anintersection point. For example, if the intersecting surface is shiny orspecular, rays are generated in the reflection direction. The ray mayaccept the color of the first object intersected, which in turn has itsintersection point tested for shadows. This reflection process isrecursively repeated until a recursion limit is reached or the potentialcontribution of subsequent bounces falls below a threshold. Rays canalso be generated in the direction of refraction for transparent solidobjects, and again recursively evaluated. See Akenine-Möller et al.,cited above. Ray tracing technology thus allows a graphics system todevelop physically correct reflections and shadows that are not subjectto the limitations and artifacts of scan conversion techniques.

Traversal Coprocessor

The basic task the traversal coprocessor performs is to test a rayagainst all primitives (commonly triangles in one embodiment) in thescene and report either the closest hit (according to distance measuredalong the ray) or simply the first (not necessarily closest) hitencountered, depending upon use case. The naïve algorithm would be anO(n) brute-force search. By pre-processing the scene geometry andbuilding a suitable acceleration data structure in advance, however, itis possible to reduce the average-case complexity to O(log n). In raytracing, the time for finding the closest (or for shadows, any)intersection for a ray is typically order O(log n) for n objects when anacceleration data structure is used. For example, bounding volumehierarchies (BVHs) of the type commonly used for modern ray tracingacceleration data structures typically have an O(log n) search behavior.

Bounding Volume Hierarchies

The acceleration data structure most commonly used by modern ray tracersis a bounding volume hierarchy (BVH) comprising nested axis-alignedbounding boxes (AABBs). The leaf nodes of the BVH contain the primitives(e.g., triangles) to be tested for intersection. The BVH is most oftenrepresented by a graph or tree structure data representation. In suchinstances, the traversal coprocessor may be called a “tree traversalunit” or “TTU”.

Given a BVH, ray tracing amounts to a tree search where each node in thetree visited by the ray has a bounding volume for each descendent branchor leaf, and the ray only visits the descendent branches or leaves whosecorresponding bound volume it intersects. In this way, only a smallnumber of primitives must be explicitly tested for intersection, namelythose that reside in leaf nodes intersected by the ray. In the examplenon-limiting embodiments, the traversal coprocessor accelerates bothtree traversal (including the ray-volume tests) and ray-primitive tests.As part of traversal, the traversal coprocessor can also handle“instance transforms”—transforming a ray from world-space coordinatesinto the coordinate system of an instanced mesh (object space) e.g., inorder to avoid the computational complexity of transforming theprimitive vertices into world space. It can do so in a MIMD(multiple-instruction, multiple data) fashion, meaning that the rays arehandled independently once inside the traversal coprocessor.

Example Non-Limiting Real Time Interactive Ray Tracing System

FIG. 1 illustrates an example real time ray interactive tracing graphicssystem 100 for generating images using three dimensional (3D) data of ascene or object(s). System 100 includes an input device 110, aprocessor(s) 120, a graphics processing unit(s) (GPU(s)) 130, memory140, and a display(s) 150. The system shown in FIG. 1 can take on anyform factor including but not limited to a personal computer, a smartphone or other smart device, a video game system, a wearable virtual oraugmented reality system, a cloud-based computing system, avehicle-mounted graphics system, a system-on-a-chip (SoC), etc.

The processor 120 may be a multicore central processing unit (CPU)operable to execute an application in real time interactive response toinput device 110, the output of which includes images for display ondisplay 150. Display 150 may be any kind of display such as a stationarydisplay, a head mounted display such as display glasses or goggles,other types of wearable displays, a handheld display, a vehicle mounteddisplay, etc. For example, the processor 120 may execute an applicationbased on inputs received from the input device 110 (e.g., a joystick, aninertial sensor, an ambient light sensor, etc.) and instruct the GPU 130to generate images showing application progress for display on thedisplay 150.

Based on execution of the application on processor 120, the processormay issue instructions for the GPU 130 to generate images using 3D datastored in memory 140. The GPU 130 includes specialized hardware foraccelerating the generation of images in real time. For example, the GPU130 is able to process information for thousands or millions of graphicsprimitives (polygons) in real time due to the GPU's ability to performrepetitive and highly-parallel specialized computing tasks such aspolygon scan conversion much faster than conventional software-drivenCPUs. For example, unlike the processor 120, which may have multiplecores with lots of cache memory that can handle a few software threadsat a time, the GPU 130 may include hundreds or thousands of processingcores or “streaming multiprocessors” (SMs) 132 running in parallel.

In one example embodiment, the GPU 130 includes a plurality ofprogrammable streaming multiprocessors (SMs) 132, and a hardware-basedgraphics pipeline including a graphics primitive engine 134 and a rasterengine 136. These components of the GPU 130 are configured to performreal-time image rendering using a technique called “scan conversionrasterization” to display three-dimensional scenes on a two-dimensionaldisplay 150. In rasterization, geometric building blocks (e.g., points,lines, triangles, quads, meshes, etc.) of a 3D scene are mapped topixels of the display (often via a frame buffer memory).

The GPU 130 converts the geometric building blocks (i.e., polygonprimitives such as triangles) of the 3D model into pixels of the 2Dimage and assigns an initial color value for each pixel. The graphicspipeline may apply shading, transparency, texture and/or color effectsto portions of the image by defining or adjusting the color values ofthe pixels. The final pixel values may be anti-aliased, filtered andprovided to the display 150 for display. Many software and hardwareadvances over the years have improved subjective image quality usingrasterization techniques at frame rates needed for real-time graphics(i.e., 30 to 60 frames per second) at high display resolutions such as4096×2160 pixels or more on one or multiple displays 150.

Traversal Coprocessor Addition to Architecture

To enable the GPU 130 to perform ray tracing in real time in anefficient manner, the GPU is provided with traversal coprocessor 138coupled to one or more SMs 132. The traversal coprocessor 138 includeshardware components configured to perform operations commonly utilizedin ray tracing algorithms. A goal of the traversal coprocessor 138 is toaccelerate operations used in ray tracing to such an extent that itbrings the power of ray tracing to real-time graphics application (e.g.,games), enabling high-quality shadows, reflections, and globalillumination. As discussed in more detail below, the result of thetraversal coprocessor 138 may be used together with or as an alternativeto other graphics related operations performed in the GPU 130.

In the example architecture shown, the new hardware component called a“traversal coprocessor” 138 is used to accelerate certain tasksincluding but not limited to ray tracing. Ray tracing refers to castinga ray into a scene and determining whether and where that ray intersectsthe scene's geometry. This basic ray tracing visibility test is thefundamental primitive underlying a variety of rendering algorithms andtechniques in computer graphics. For example, ray tracing can be usedtogether with or as an alternative to rasterization and z-buffering forsampling scene geometry. It can also be used as an alternative to (or incombination with) environment mapping and shadow texturing for producingmore realistic reflection, refraction and shadowing effects than can beachieved via texturing techniques or other raster “hacks”. To overcomelimitations in image quality that can be achieved with rasterization,system 100 can also generate entire images or parts of images using raytracing techniques. Ray tracing may also be used as the basic primitiveto accurately simulate light transport in physically-based renderingalgorithms such as path tracing, photon mapping, Metropolis lighttransport, and other light transport algorithms.

More specifically, SMs 132 and the traversal coprocessor 138 maycooperate to cast rays into a 3D model and determine whether and wherethat ray intersects the model's geometry. Ray tracing directly simulateslight traveling through a virtual environment or scene. The results ofthe ray intersections together with surface texture, viewing direction,and/or lighting conditions are used to determine pixel color values. Raytracing performed by SMs 132 working with traversal coprocessor 138allows for computer-generated images to capture shadows, reflections,and refractions in ways that can be indistinguishable from photographsor video of the real world. Since ray tracing techniques are even morecomputationally intensive than rasterization due in part to the largenumber of rays that need to be traced, the traversal coprocessor 138 iscapable of accelerating in hardware certain of the morecomputationally-intensive aspects of that process.

In the example non-limiting technology herein, traversal coprocessor 138accelerates both ray-box tests and ray-primitive tests. As part oftraversal, it can also handle at least one level of instance transforms,transforming a ray from world-space coordinates into the coordinatesystem of an instanced mesh. In the example non-limiting embodiments,the traversal coprocessor 138 does all of this in MIMD fashion, meaningthat rays are handled independently once inside the traversalcoprocessor.

In the example non-limiting embodiments, the traversal coprocessor 138operates as a servant (coprocessor) to the SMs (streamingmultiprocessors) 132. In other words, the traversal coprocessor 138 inexample non-limiting embodiments does not operate independently, butinstead follows the commands of the SMs 132 to perform certaincomputationally-intensive ray tracing related tasks much moreefficiently than the SMs 132 could perform themselves.

In the examples shown, the traversal coprocessor 138 receives commandsvia SM 132 instructions and writes results back to an SM register file.For many common use cases (e.g., opaque triangles with at most one levelof instancing), the traversal coprocessor 138 can service the raytracing query without further interaction with the SM 132. Morecomplicated queries (e.g., involving alpha-tested triangles, primitivesother than triangles, or multiple levels of instancing) may requiremultiple round trips. In addition to tracing rays, the traversalcoprocessor 138 is capable of performing more general spatial querieswhere an AABB or the extruded volume between two AABBs (which we call a“beam”) takes the place of the ray. Thus, while the traversalcoprocessor 138 is especially adapted to accelerate ray tracing relatedtasks, it can also be used to perform tasks other than ray tracing.

In addition to the traversal coprocessor 138, the example non-limitingtechnology used to support the system 100 of FIG. 1 provides additionalaccelerated ray tracing enhancements to a number of units as well as asubstantial effort devoted to BVH construction. BVH construction neednot be hardware accelerated (although it may be in some non-limitingembodiments) but could instead be implemented using highly-optimizedsoftware routines running on SMs 132 and/or CPU 120 and/or otherdevelopment systems e.g., during development of an application. Thefollowing exposition describes, among other things, software-visiblebehavior of the traversal coprocessor 138, interfaces to surroundingunits (SMs 132 and the memory subsystem), and additional features thatare part of a complete ray-tracing solution such as certain enhancementsto the group of SMs 132 and the memory caching system.

Traversing an Acceleration Data Structure

A good way to accelerate ray tracing is to use an acceleration datastructure. The acceleration data structure represents the 3D model of anobject or a scene in a manner that will help assist in quickly decidingwhich portion of the object a particular ray is likely to intersect andquickly rejecting large portions of the scene the ray will notintersect. A bounding volume hierarchy (BVH) data structure is one typeof acceleration data structure which can help reduce the number ofintersections to test. The BVH data structure represents a scene orobject with a bounding volume and subdivides the bounding volume intosmaller and smaller bounding volumes terminating in leaf nodescontaining geometric primitives. The bounding volumes are hierarchical,meaning that the topmost level encloses the level below it, that levelencloses the next level below it, and so on. In one embodiment, leafnodes can potentially overlap other leaf nodes in the bounding volumehierarchy.

To illustrate how a bounding volume hierarchy works, FIGS. 2A-2G show ateapot recursively subdivided into smaller and smaller hierarchicalbounding volumes. FIG. 2A shows a teapot object, and FIG. 2B shows abounding volume 202 (in this case a box, cube or rectangularparallelepiped) enclosing the whole teapot. The bounding volume 202,which can be efficiently defined by its vertices, provides an indicationof the spatial location of the object and is typically dimensioned to bejust slightly larger than the object.

The first stage in acceleration structure construction acquires thebounding boxes of the referenced geometry. This is achieved by executingfor each geometric primitive in an object a bounding box procedure thatreturns a conservative axis-aligned bounding box for its input primitivesuch as box 202 shown in FIG. 2B. Using these bounding boxes aselementary primitives for the acceleration structures provides thenecessary abstraction to trace rays against arbitrary user-definedgeometry (including several types of geometry within a singlestructure). Because in FIG. 2B the bounding volume 202 is larger thanand completely contains the teapot, a ray that does not intersectbounding volume cannot intersect the teapot, although a ray that doesintersect the bounding volume may or may not intersect the teapot.Because the bounding volume 202 is readily defined by the x,y,zcoordinates of its vertices in 3D space and a ray is defined by itsx,y,z coordinates in 3D space, the ray-bounding volume test to determinewhether a ray intersects the bounding volume 202 is straightforward(although some transform may be used to adjust to different coordinatesystems, as will be explained below).

FIG. 2C, shows the bounding volume 202 subdivided into smaller containedbounding volumes. While the subdivision scheme shown here for purposesof illustration is a so-called 8-ary subdivision or “octree” in whicheach volume is subdivided into eight smaller volumes of uniform size,many other spatial hierarchies and subdivision schemes are known such asa binary tree, a four-ary tree, a k-d tree, a binary space partitioning(BSP) tree, and a bounding volume hierarchy (BVH) tree. See e.g., U.S.Pat. No. 9,582,607.

Each of the subdivided bounding volumes shown in FIG. 2C can be stillfurther subdivided. FIG. 2D shows one of the subdivided volumes 204 ofFIG. 2C being further subdivided to provide additional subdividedencapsulated bounding volumes. As shown in FIG. 2D, some of thesubdivided bounding volumes include portions of the teapot and some donot. Volumes that do not contain a portion of the teapot are not furthersubdivided because the further subdivisions provide no further spatialinformation about the teapot. Already subdivided bounding volumes thatdo include at least one portion of the teapot can be still furtherrecursively subdivided—like the emergence of each of a succession oflittler and littler cats from the hats of Dr. Seuss's' The Cat In TheHat Comes Back (1958). The portions of the space within bounding volume202 that contain geometry are recursively subdivided to permit thetraversal coprocessor 138 to use the volumetric subdivisions toefficiently discover where the geometry is located relative to any givenray. It can be noted that while a spatial or active subdivision of thevolume is possible, many implementations will create the hierarchicalstructure defining volumes and subvolumes ahead of time. In such cases,the builder may often build the hierarchy up from individual trianglesand not down from the whole scene. Building up means you do not need todetermine if some subdivided volume contains anything since bydefinition it contains what is below it in a hierarchy of volumetricsubdivisions.

FIG. 2E shows a further such subdivision of bounding volume 204 into afurther smaller contained bounding volume 206 containing in this examplejust the spout of the teapot plus another surface on the wall of theteapot, and FIG. 2F shows an additional subdivision of bounding volume206 into still smaller contained subdivision 208 encapsulating the endof the teapot's spout. Depending on the way the BVH is constructed,bounding volume 208 can be further and further subdivided as desired—andtraversal coprocessor 138 enables the FIG. 1 system 100 to efficientlytraverse the BVH down to any arbitrary subdivision level. The number andconfigurations of recursive subdivisions will depend on the complexityand configuration of the 3D object being modeled as well as otherfactors such as desired resolution, distance of the object from theviewpoint, etc.

At some level of subdivision (which can be different levels fordifferent parts of the BVH), the traversal coprocessor 138 encountersgeometry making up the encapsulated object being modeled. Using theanalogy of a tree, the successive volumetric subdivisions are the trunk,branches, boughs and twigs, and the geometric is finally revealed at thevery tips of the tree, namely the leaves. In this case, FIG. 2G showsthe surface of the teapot's spout defined by an example mesh ofgeometric primitives. The geometric primitives shown are triangles butother geometric primitives, such as quads, lines, rectangles, quadrics,patches, or other geometric primitives known to those familiar with thestate of the art, may be used (in one embodiment, such other types ofprimitives may be expressed as or converted into triangles). Thegeometric primitives in the mesh represent the shape of the 3D surfaceof the object being modeled. The example shown here is a mesh, butbounded geometry can include discontinuous geometry such as particlesthat may not be connected. In the example non-limiting embodiments, thetraversal coprocessor 138 also accelerates ray intersection tests withthis geometry to quickly determine which triangles are hit by any givenray. Determining ray-primitive intersections involves comparing thespatial xyz coordinates of the vertices of each primitive with the xyzcoordinates of the ray to determine whether the ray and the surface theprimitive defines occupy the same space. The ray-primitive intersectiontest can be computationally intensive because there may be manytriangles to test. For example, in the mesh shown in FIG. 2G, the spoutof the teapot alone is made up of over a hundred triangles—although itmay be more efficient in some implementations to further volumetricallysubdivide and thereby limit the number of triangles in any such “leafnode” to something like 16 or fewer.

As discussed above, ray tracing procedures determine what geometricprimitives of a scene are intersected by a ray. However, due to thelarge number of primitives in a 3D scene, it may not be efficient orfeasible to test every geometric primitive for an intersection.Acceleration data structures, such as BVH, allow for quick determinationas to which bounding volumes can be ignored, which bounding volumes maycontain intersected geometric primitives, and which intersectedgeometric primitives matter for visualization and which do not.

Ray Intersection Testing

FIGS. 3A-3C illustrate ray tracing applied to the FIG. 2G boundingvolume 208 including triangle mesh 320. FIG. 3A shows a ray 302 in avirtual space including bounding volumes 310 and 315. To determinewhether the ray 302 intersects one or more triangles in the mesh 320,each triangle could be directly tested against the ray 302. But toaccelerate the process (since the object could contain many thousands oftriangles), the ray 302 is first tested against the bounding volumes 310and 315. If the ray 302 does not intersect a bounding volume, then itdoes not intersect any triangles inside of the bounding volume and alltriangles inside the bounding volume can be ignored for purposes of thatray. Because in FIG. 3A the ray 302 misses bounding volume 310, thetriangles of mesh 320 within that bounding volume need not be tested forintersection. While bounding volume 315 is intersected by the ray 302,bounding volume 315 does not contain any geometry and so no furthertesting is required.

On the other hand, if a ray such as ray 304 shown in FIG. 3B intersectsa bounding volume 310 that contains geometry, then the ray may or maynot intersect the geometry inside of the bounding volume so furthertests need to be performed on the geometry itself to find possibleintersections. Because the rays 304, 306 in FIGS. 3B and 3C intersect abounding volume 310 that contains geometry, further tests need to beperformed to determine whether any (and which) of the primitives insideof the bounding volume are intersected. In FIG. 3B, further testing ofthe intersections with the primitives would indicate that even thoughthe ray 304 passes through the bounding volume 310, it does notintersect any of the primitives the bounding volume encloses(alternatively, as mentioned above, bounding volume 310 could be furthervolumetrically subdivided so that a bounding volume intersection testcould be used to reveal that the ray does not intersect any geometry ormore specifically which primitives the ray may intersect).

FIG. 3C shows a situation in which the bounding volume 310 intersectedby ray 306 and contains geometry that ray 306 intersects. Traversalcoprocessor 138 tests the intersections between the ray 306 and theindividual primitives to determine which primitives the ray intersects.

Ray Tracing Operations

FIG. 4 is a flowchart summarizing example ray tracing operations thetraversal coprocessor 138 performs as described above in cooperationwith SM(s) 132. The FIG. 4 operations are performed by traversalcoprocessor 138 in cooperation with its interaction with an SM 132. Thetraversal coprocessor 138 may thus receive the identification of a rayfrom the SM 132 and traversal state enumerating one or more nodes in oneor more BVH's that the ray must traverse. The traversal coprocessor 138determines which bounding volumes of a BVH data structure the rayintersects (the “ray-complet” test 512) and subsequently whether the rayintersects one or more primitives in the intersected bounding volumesand which triangles are intersected (the “ray-primitive test” 520). Inexample non-limiting embodiments, “complets” (compressed treelets)specify root or interior nodes (i.e., volumes) of the bounding volumehierarchy with children that are other complets or leaf nodes of asingle type per complet.

First, the traversal coprocessor 138 inspects the traversal state of theray. If a stack the traversal coprocessor 138 maintains for the ray isempty, then traversal is complete. If there is an entry on the top ofthe stack, the traversal co-processor 138 issues a request to the memorysubsystem to retrieve that node. The traversal co-processor 138 thenperforms a bounding box test 512 to determine if a bounding volume of aBVH data structure is intersected by a particular ray the SM 132specifies (step 512, 514). If the bounding box test determines that thebounding volume is not intersected by the ray (“No” in step 514), thenthere is no need to perform any further testing for visualization andthe traversal coprocessor 138 can return this result to the requestingSM 132. This is because if a ray misses a bounding volume (as in FIG. 3Awith respect to bounding volume 310), then the ray will miss all othersmaller bounding volumes inside the bounding volume being tested and anyprimitives that bounding volume contains.

If the bounding box test performed by the traversal coprocessor 138reveals that the bounding volume is intersected by the ray (“Yes” inStep 514), then the traversal coprocessor determines if the boundingvolume can be subdivided into smaller bounding volumes (step 518). Inone example embodiment, the traversal coprocessor 138 isn't necessarilyperforming any subdivision itself. Rather, each node in the BVH has oneor more children (where each child is a leaf or a branch in the BVH).For each child, there is a bounding volume and a pointer that leads to abranch or a leaf node. When a ray processes a node using traversalcoprocessor 138, it is testing itself against the bounding volumes ofthe node's children. The ray only pushes stack entries onto its stackfor those branches or leaves whose representative bounding volumes werehit. When a ray fetches a node in the example embodiment, it doesn'ttest against the bounding volume of the node—it tests against thebounding volumes of the node's children. The traversal coprocessor 138pushes nodes whose bounding volumes are hit by a ray onto the ray'straversal stack in an order determined by ray configuration. Forexample, it is possible to push nodes onto the traversal stack in theorder the nodes appear in memory, or in the order that they appear alongthe length of the ray, or in some other order. If there are furthersubdivisions of the bounding volume (“Yes” in step 518), then thosefurther subdivisions of the bounding volume are accessed and thebounding box test is performed for each of the resulting subdividedbounding volumes to determine which subdivided bounding volumes areintersected by the ray and which are not. In this recursive process,some of the bounding volumes may be eliminated by test 514 while otherbounding volumes may result in still further and further subdivisionsbeing tested for intersection by traversal coprocessor 138 recursivelyapplying steps 512-518.

Once the traversal coprocessor 138 determines that the bounding volumesintersected by the ray are leaf nodes (“No” in step 518), the traversalcoprocessor performs a primitive (e.g., triangle) intersection test 520to determine whether the ray intersects primitives in the intersectedbounding volumes and which primitives the ray intersects. The traversalcoprocessor 138 thus performs a depth-first traversal of intersecteddescendent branch nodes until leaf nodes are reached. The traversalcoprocessor 138 processes the leaf nodes. If the leaf nodes areprimitive ranges, the traversal coprocessor 138 tests them against theray. If the leaf nodes are instance nodes, the traversal coprocessor 138applies the instance transform. If the leaf nodes are item ranges, thetraversal coprocessor 138 returns them to the requesting SM 132. In theexample non-limiting embodiments, the SM 132 can command the traversalcoprocessor 138 to perform different kinds of ray-primitive intersectiontests and report different results depending on the operations comingfrom an application (or an software stack the application is running on)and relayed by the SM to the TTU. For example, the SM 132 can commandthe traversal coprocessor 138 to report the nearest visible primitiverevealed by the intersection test, or to report all primitives the rayintersects irrespective of whether they are the nearest visibleprimitive. The SM 132 can use these different results for differentkinds of visualization. Once the traversal coprocessor 138 is doneprocessing the leaf nodes, there may be other branch nodes (pushedearlier onto the ray's stack) to test.

Multiple Intersections

In more detail, as shown in FIG. 3C, any given ray may intersectmultiple primitives within a bounding volume. Whether the rayintersection within a given primitive matters for visualization dependson the properties and position of that primitive as well as thevisualization procedures the SM 132 is performing. For example,primitives can be opaque, transparent or partially transparent (i.e.,translucent). Opaque primitives will block a ray from passing throughthe primitive because the eye cannot see through the primitive's opaquesurface. Transparent primitives will allow the ray to pass through(because the eye can see through the transparent primitive) but thesituation may be more complex. For example, transparent primitives mayhave specular properties that cause some portion of the ray to reflect(think of reflection from a window pane) and the rest of the ray to passthrough. Other transparent primitives are used to provide a surface ontowhich a texture is mapped. For example, each individual leaf of a treemay be modeled by a transparent primitive onto which an image of theleaf is texture mapped.

FIGS. 5A-5C illustrate some of these scenarios using an example of threetriangles assumed to be in the same bounding volume and each intersectedby a ray. FIG. 5A illustrates a ray directed towards these threetriangles, with the first triangle the ray encounters relative to theviewpoint being opaque. Because the “front” (from the standpoint of thedirection of the ray from the eye) intersected triangle is opaque, thattriangle will block the ray so the ray will not reach the othertriangles even through it spatially intersects them. In this example,the triangles “behind” the opaque triangle from the viewpoint can beignored (culled) after the intersection of the opaque triangle isidentified because the “front”, opaque triangle hides the othertriangles from the user's view along the ray. Culling is indicated bydotted lines in FIGS. 5A-5C. In this case, the traversal coprocessor 138may only need to report the identification of the first, opaque triangleto the SM 132.

FIG. 5B illustrates a ray directed towards the same three triangles butnow the nearest visible triangle is partially transparent rather thanopaque. Because the nearest visible intersected triangle is at leastpartially transparent, the ray may pass through it to hit the opaquetriangle behind it. In this case, the opaque triangle will be visiblethrough the partially transparent triangle but will block the user'sview of the third triangle along the ray. Here, the traversalcoprocessor 138 may report the identification of both front triangles tothe SM 132 but not report the third, culled triangle even though the rayspatially intersects that third triangle. Order of discovery may matterhere. In the case of an alpha and opaque triangle, if the opaque wasfound first, the traversal coprocessor 138 returns the opaque triangleto the SM 132 with traversal state that will resume testing at the alphatriangle. While there is an implication here that the alpha meanstransparent, it really means “return me to the SM 132 and let the SMdetermine how to handle it.” For example, an alpha triangle might betrimmed according to a texture or function so that portions of thetriangle are cut away (i.e., absent, not transparent). The traversalcoprocessor 138 does not know how the SM 132 will handle the alphatriangles (i.e., it does not handle transparent triangles differentlyfrom trimmed triangles). Thus, alpha triangles may or may not block ortint the light arriving from points beyond them along the ray, and inexample embodiments, they require SM 132 intervention tohandle/determine those things.

FIG. 5C illustrates a scenario in which the first two triangles the rayencounters are partially transparent. Because the first and secondintersected triangles are at least partially transparent, the ray willpass through the first and second triangles to impinge upon thealso-intersecting third opaque triangle. Because third intersectedtriangle is opaque, it will block the ray, and the ray will not impingeupon any other triangles behind the third triangle even though they maybe spatially intersected by it. In this case, the traversal coprocessor138 may report all three triangles to the SM 132 but need not report anyfurther triangles behind the opaque triangle because the opaque triangleblocks the ray from reaching those additional triangles.

In some modes, however, the SM 132 may need to know the identities ofall triangles the ray intersects irrespective of whether they are opaqueor transparent. In those modes, the traversal coprocessor 138 can simplyperform the intersection test and return the identities of all trianglesthe ray spatially intersects (in such modes, the traversal coprocessorwill return the same intersection results for all three scenarios shownin FIGS. 5A-5C) and allow the SM 132 to sort it out—or in some casescommand the traversal coprocessor 138 to do more tests on these sametriangles.

As will be discussed in more detail below, when a ray intersects anopaque triangle, the traversal coprocessor 138 can in certain operationsbe programmed to reduce the length of the ray being tested to thelocation of the opaque triangle intersection so it will not report anytriangles “behind” the intersected triangle. When a partiallytransparent triangle is determined to be intersected by a ray, thetraversal coprocessor 138 will return a more complete list of trianglesthe ray impinges upon for purposes of visualization, and the requestingSM 132 may perform further processing to determine whether, based forexample any texture or other properties of the triangle, the ray will beblocked, passed or partially passed and partially reflected. In exampleembodiments, the traversal coprocessor 138 does not have access totexture properties of triangles and so does not attempt to determinevisualization with respect to those properties.

Textures or Other Surface Modifications

For example, FIGS. 6A and 6B show a transparent triangle 610 with atexture 615 of a leaf applied to the triangle. One could think of atriangle made of Plexiglas with a decal of a leaf applied to it. Asshown in FIG. 6A, the ray 620 intersects the transparent triangle 610 ata point that is outside the applied texture 615. Because the ray 620intersects the triangle outside the applied texture 615, the texturewill not block the ray 620 and the ray will pass through the transparenttriangle 610 without obstruction. This is like being able to see throughthe parts of the Plexiglas triangle that are not covered by the leafdecal. Note that in one example embodiment, the SM 132 makes thevisibility determination since the traversal coprocessor 138 does notnecessarily have access to information concerning the leaf decal. Thetraversal coprocessor 138 helps the SM 132 by returning to the SM theidentification of the triangle that the ray intersects along withinformation concerning the properties of that triangle.

In FIG. 6B, the ray 630 intersects the transparent triangle where thetexture 615 is applied. SM 132 will determine whether subsequenttraversal by the traversal coprocessor 138 is necessary or not based onwhether the texture 615 will block the ray 630 or allow the ray 630 topass through. If the ray 630 is blocked by the texture 615, othertriangles behind the transparent triangle 610, which may have otherwisebeen intersected by the ray 630, will be obstructed by the texture andnot contribute to visualization along the ray. In the examplenon-limiting embodiments herein, the traversal coprocessor 138 does nothave access to texture information and so it does not attempt toaccelerate this determination. Traversal coprocessor 138 may for examplereturn to the requesting SM 132 all intersections between the ray andthe various triangles within the object, and the SM may then use thegraphics primitive engine 134 to make further ray tracing visualizationdeterminations. In other example embodiments, traversal coprocessor 138could accelerate some or all of these tests by interacting with thetexture mapping unit and other portions of the 3D graphics pipelinewithin graphics primitive engine 134 to make the necessary visualizationdeterminations.

Coordinate Transforms

FIGS. 2A-3C involve only a single object, namely a teapot. Just as theroom you are in right now contains multiple objects, most 3D scenescontain many objects. For example, a 3D scene containing a teapot willlikely also contain a cup, a saucer, a milk pitcher, a spoon, a sugarbowl, etc. all sitting on a table. In 3D graphics, each of these objectsis typically modelled independently. The graphics system 100 then usescommands from the processor 120 to put all the models together indesired positions, orientations and sizes into the common scene forpurposes of visualization (just as you will set and arrange the tablefor serving tea). What this means is that the SM 132 may commandtraversal processor 138 to analyze the same ray with respect to multipleobjects in the scene. However, the fact that each of these objects willbe transformed in position, orientation and size when placed into thecommon scene is taken into account and accelerated by the traversalcoprocessor 138. In non-limiting example embodiments, the transform fromworld-to-object space is stored in the world space BVH along with aworld-space bounding box. The traversal coprocessor 138 accelerates thetransform process by transforming the ray from world (scene) space intoobject space for purposes of performing the tests shown in FIG. 4. Inparticular, since the transformation of the geometry from object spaceinto world (scene) space is computationally intensive, thattransformation is left to the graphics pipeline graphics primitiveengine 134 and/or raster engine 136 to perform as part of rasterization.The traversal coprocessor 138 instead transforms a given ray from worldspace to the coordinate system of each object defined by an accelerationdata structure and performs its tests in object space.

FIGS. 7A and 7B illustrates how the traversal coprocessor 138 transformsthe same ray into three different object spaces. FIG. 7A shows threeobjects on a table: a cup, a teapot and a pitcher. These three objectsand a table comprise a scene, which exists in world space. A ray thatalso is defined in world space emanates from the viewpoint andintersects each of the three objects.

FIG. 7B shows each of the three objects as defined in object spaces.Each of these three objects is defined by a respective model that existsin a respective object space. The traversal coprocessor 138 in examplenon-limiting embodiments transforms the ray into the object space ofeach object before performing the intersection tests for that object.This “instance transform” saves the computational effort of transformingthe geometry of each object and the associated volumetric subdivisionsof the acceleration data structure from object space to world space forpurposes of the traversal coprocessor 138 performing intersection tests.

The requesting SM 132 keeps track of which objects are in front of whichother objects with respect to each individual ray and resolvesvisibility in cases where one object hides another object, casts ashadow on another object, and/or reflects light toward another object.The requesting SM 132 can use the traversal processor 138 to accelerateeach of these tests.

Example Tree BVH Acceleration Data Structure

FIGS. 8A and 8B show a recursively-subdivided bounding volume of a 3Dscene (FIG. 8A) and a corresponding tree data structure (FIG. 8B) thatmay be accessed by the traversal coprocessor 138 and used forhardware-accelerated operations performed by traversal coprocessor. Thedivision of the bounding volumes may be represented in a hierarchicaltree data structure with the large bounding volume shown in FIG. 2Brepresented by a parent node of the tree and the smaller boundingvolumes represented by children nodes of the tree that are contained bythe parent node. The smallest bounding volumes are represented as leafnodes in the tree and identify one or more geometric primitivescontained within these smallest bounding volumes.

The tree data structure may be stored in memory outside of the traversalcoprocessor 138 and retrieved based on queries the SMs 132 issue to thetraversal coprocessor 138. The tree data structure includes a pluralityof nodes arranged in a hierarchy. The root nodes N1 of the treestructure correspond to bounding volume N1 enclosing all of thetriangles O1-O8. The root node N1 may identify the vertices of thebounding volume N1 and children nodes of the root node.

In FIG. 8A, bounding volume N1 is subdivided into bounding volumes N2and N3. Children nodes N2 and N3 of the tree structure of FIG. 8Bcorrespond to and represent the bounding volumes N2 and N3 shown in FIG.8A. The children nodes N2 and N3 in the tree data structure identify thevertices of respective bounding volumes N2 and N3 in space. Each of thebounding volumes N2 and N3 is further subdivided in this particularexample. Bounding volume N2 is subdivided into contained boundingvolumes N4 and N5. Bounding volume N3 is subdivided into containedbounding volumes N6 and N7. Bounding volume N7 include two boundingvolumes N8 and N9. Bounding volume N8 includes the triangles O7 and O8,and bounding volume N9 includes leaf bounding volumes N10 and N11 as itschild bounding volumes. Leaf bounding volume N10 includes a primitiverange (e.g., triangle range) O10 and leaf bounding volume N11 includesan item range O9. Respective children nodes N4, N5, N6, N8, N10 and N11of the FIG. 8B tree structure correspond to and represent the FIG. 8Abounding volumes N4, N5, N6, N8, N10 and N11 in space.

The FIG. 8B tree is only three to six levels deep so that volumes N4,N5, N6, N8, N10 and N11 constitute “leaf nodes”—that is, nodes in thetree that have no child nodes. FIG. 8A shows that each of leaf nodebounding volumes N4, N5, N6, and N8, contains two triangles of thegeometry in the scene. For example, volumetric subdivision N4 containstriangles O1 & O2; volumetric subdivision N5 contains triangles O3 & O4;volumetric subdivision N6 contains trials O5 & O6; and volumetricsubdivision N8 contains triangles O7 & O8. The tree structure shown inFIG. 8B represents these leaf nodes N4, N5, N6, and N7 by associatingthem with the appropriate ones of triangles O1-O8 of the scene geometry.To access this scene geometry, the traversal coprocessor 138 traversesthe tree data structure of FIG. 8B down to the leaf nodes. In general,different parts of the tree can and will have different depths andcontain different numbers of triangles. Leaf nodes associated withvolumetric subdivisions that contain no geometry need not be explicitlyrepresented in the tree data structure (i.e., the tree is “trimmed”).

According to some embodiments, the subtree rooted at N7 may represent aset of bounding volumes or BVH that is defined in a different coordinatespace than the bounding volumes corresponding to nodes N1-N3. Whenbounding volume N7 is in a different coordinate space from its parentbounding volume N3, an instance node N7′ which provides the raytransformation necessary to traverse the subtree rooted at N7, mayconnect the rest of the tree to the subtree rooted at N7. Instance nodeN7′ connects the bounding volume or BVH corresponding to nodes N1-N3,with the bounding volumes or BVH corresponding to nodes N7 etc. bydefining the transformation from the coordinate space of N1-N3 (e.g.,world space) to the coordinate space of N7 etc. (e.g., object space).

The Internal Structure and Operation of Traversal Coprocessor 138

FIG. 9 shows an example simplified block diagram of traversalcoprocessor 138 including hardware configured to perform acceleratedtraversal operations as described above (a still more detailedimplementation of this traversal coprocessor 138 is described below).Because the traversal coprocessor 138 shown in FIG. 9 is adapted totraverse tree-based acceleration data structures such as shown in FIGS.8A, 8B, it may also be called a “tree traversal unit” or “TTU” 700 (the700 reference number is used to refer to the more detailed non-limitingimplementation of traversal coprocessor 138 shown in FIG. 1). Treetraversal operations may include, for example, determining whether a rayintersects bounding volumes and/or primitives of a tree data structure(e.g., a BVH tree), which tests may involve transforming the ray intoobject space.

The TTU 700 includes dedicated hardware to determine whether a rayintersects bounding volumes and dedicated hardware to determine whethera ray intersects primitives of the tree data structure. In someembodiments, the TTU 700 may perform a depth-first traversal of abounding volume hierarchy using a short stack traversal withintersection testing of supported leaf node primitives and mid-traversalreturn of alpha primitives and unsupported leaf node primitives (items).The intersection of primitives will be discussed with reference totriangles, but other geometric primitives may also be used.

In more detail, TTU 700 includes an intersection management block 722, aray management block 730 and a stack management block 740. Each of theseblocks (and all of the other blocks in FIG. 9) may constitute dedicatedhardware implemented by logic gates, registers, hardware-embedded lookuptables or other combinatorial logic, etc.

The ray management block 730 is responsible for managing informationabout and performing operations concerning a ray specified by an SM 132to the ray management block. The stack management block 740 works inconjunction with traversal logic 712 to manage information about andperform operations related to traversal of a BVH acceleration datastructure. Traversal logic 712 is directed by results of a ray-complettest block 710 that tests intersections between the ray indicated by theray management block 730 and volumetric subdivisions represented by theBVH, using instance transforms as needed. The ray-complet test block 710retrieves additional information concerning the BVH from memory 140 viaan L0 complet cache 752 that is part of the TTU 700. The results of theray-complet test block 710 informs the traversal logic 712 as to whetherfurther recursive traversals are needed. The stack management block 740maintains stacks to keep track of state information as the traversallogic 712 traverses from one level of the BVH to another, with the stackmanagement block pushing items onto the stack as the traversal logictraverses deeper into the BVH and popping items from the stack as thetraversal logic traverses upwards in the BVH. The stack management block740 is able to provide state information (e.g., intermediate or finalresults) to the requesting SM 132 at any time the SM requests.

The intersection management block 722 manages information about andperforms operations concerning intersections between rays andprimitives, using instance transforms as needed. The ray-primitive testblock 720 retrieves information concerning geometry from memory 140 onan as-needed basis via an L0 primitive cache 754 that is part of TTU700. The intersection management block 722 is informed by results ofintersection tests the ray-primitive test and transform block 720performs. Thus, the ray-primitive test and transform block 720 providesintersection results to the intersection management block 722, whichreports geometry hits and intersections to the requesting SM 132.

A Stack Management Unit 740 inspects the traversal state to determinewhat type of data needs to be retrieved and which data path (complet orprimitive) will consume it. The intersections for the bounding volumesare determined in the ray-complet test path of the TTU 700 including oneor more ray-complet test blocks 710 and one or more traversal logicblocks 712. A complet specifies root or interior nodes of a boundingvolume. Thus, a complet may define one or more bounding volumes for theray-complet test. The ray-complet test path of the TTU 700 identifieswhich bounding volumes are intersected by the ray. Bounding volumesintersected by the ray need to be further processed to determine if theprimitives associated with the intersected bounding volumes areintersected. The intersections for the primitives are determined in theray-primitive test path including one or more ray-primitive test andtransform blocks 720 and one or more intersection management blocks 722.

The TTU 700 receives queries from one or more SMs 132 to perform treetraversal operations. The query may request whether a ray intersectsbounding volumes and/or primitives in a BVH data structure. The querymay identify a ray (e.g., origin, direction, and length of the ray) anda BVH data structure and traversal state (e.g., short stack) whichincludes one or more entries referencing nodes in one or more BoundingVolume Hierarchies that the ray is to visit. The query may also includeinformation for how the ray is to handle specific types of intersectionsduring traversal. The ray information may be stored in the raymanagement block 730. The stored ray information (e.g., ray length) maybe updated based on the results of the ray-primitive test.

The TTU 700 may request the BVH data structure identified in the queryto be retrieved from memory outside of the TTU 700. Retrieved portionsof the BVH data structure may be cached in the level-zero (L0) cache 750within the TTU 700 so the information is available for othertime-coherent TTU operations, thereby reducing memory 140 accesses.Portions of the BVH data structure needed for the ray-complet test maybe stored in a L0 complet cache 752 and portions of the BVH datastructure needed for the ray-primitive test may be stored in an L0primitive cache 754.

After the complet information needed for a requested traversal step isavailable in the complet cache 752, the ray-complet test block 710determines bounding volumes intersected by the ray. In performing thistest, the ray may be transformed from the coordinate space of thebounding volume hierarchy to a coordinate space defined relative to acomplet. The ray is tested against the bounding boxes associated withthe child nodes of the complet. In the example non-limiting embodiment,the ray is not tested against the complet's own bounding box because (1)the TTU 700 previously tested the ray against a similar bounding boxwhen it tested the parent bounding box child that referenced thiscomplet, and (2) a purpose of the complet bounding box is to define alocal coordinate system within which the child bounding boxes can beexpressed in compressed form. If the ray intersects any of the childbounding boxes, the results are pushed to the traversal logic todetermine the order that the corresponding child pointers will be pushedonto the traversal stack (further testing will likely require thetraversal logic 712 to traverse down to the next level of the BVH).These steps are repeated recursively until intersected leaf nodes of theBVH are encountered

The ray-complet test block 710 may provide ray-complet intersections tothe traversal logic 612. Using the results of the ray-complet test, thetraversal logic 712 creates stack entries to be pushed to the stackmanagement block 740. The stack entries may indicate internal nodes(i.e., a node that includes one or more child nodes) that need to befurther tested for ray intersections by the ray-complet test block 710and/or triangles identified in an intersected leaf node that need to betested for ray intersections by the ray-primitive test and transformblock 720. The ray-complet test block 710 may repeat the traversal oninternal nodes identified in the stack to determine all leaf nodes inthe BVH that the ray intersects. The precise tests the ray-complet testblock 710 performs will in the example non-limiting embodiment bedetermined by mode bits, ray operations (see below) and culling of hits,and the TTU 700 may return intermediate as well as final results to theSM 132.

The intersected leaf nodes identify primitives that may or may not beintersected by the ray. One option is for the TTU 700 to provide e.g., arange of geometry identified in the intersected leaf nodes to the SM 132for further processing. For example, the SM 132 may itself determinewhether the identified primitives are intersected by the ray based onthe information the TTU 700 provides as a result of the TTU traversingthe BVH. To offload this processing from the SM 132 and therebyaccelerate it using the hardware of the TTU 700, the stack managementblock 740 may issue requests for the ray-primitive and transform block720 to perform a ray-primitive test for the primitives withinintersected leaf nodes the TTU's ray-complet test block 710 identified.In some embodiments, the SM 132 may issue a request for theray-primitive test to test a specific range of primitives and transformblock 720 irrespective of how that geometry range was identified.

After making sure the primitive data needed for a requestedray-primitive test is available in the primitive cache 754, theray-primitive and transform block 710 may determine primitives that areintersected by the ray using the ray information stored in the raymanagement block 730. The ray-primitive test block 720 provides theidentification of primitives determined to be intersected by the ray tothe intersection management block 722.

The intersection management block 722 can return the results of theray-primitive test to the SM 132. The results of the ray-primitive testmay include identifiers of intersected primitives, the distance ofintersections from the ray origin and other information concerningproperties of the intersected primitives. In some embodiments, theintersection management block 722 may modify an existing ray-primitivetest (e.g., by modifying the length of the ray) based on previousintersection results from the ray-primitive and transform block 710.

The intersection management block 722 may also keep track of differenttypes of primitives. For example, the different types of trianglesinclude opaque triangles that will block a ray when intersected andalpha triangles that may or may not block the ray when intersected ormay require additional handling by the SM. Whether a ray is blocked ornot by a transparent triangle may for example depend on texture(s)mapped onto the triangle, area of the triangle occupied by the texture(see FIGS. 6A and 6B) and the way the texture modifies the triangle. Forexample, transparency (e.g., stained glass) in some embodiments requiresthe SM 132 to keep track of transparent object hits so they can besorted and shaded in ray-parametric order, and typically don't actuallyblock the ray. Meanwhile, alpha “trimming” allows the shape of theprimitive to be trimmed based on the shape of a texture mapped onto theprimitive—for example, cutting a leaf shape out of a triangle. (Notethat in raster graphics, transparency is often called “alpha blending”and trimming is called “alpha test”). In other embodiments, the TTU 700can push transparent hits to queues in memory for later handling by theSM 132 and directly handle trimmed triangles by sending requests to thetexture unit. Each triangle may include a designator to indicate thetriangle type. The intersection management block 722 is configured tomaintain a result queue for tracking the different types of intersectedtriangles. For example, the result queue may store one or moreintersected opaque triangle identifiers in one queue and one or moretransparent triangle identifiers in another queue.

For opaque triangles, the ray intersection can be fully determined inthe TTU 700 because the area of the opaque triangle blocks the ray fromgoing past the surface of the triangle. For transparent triangles, rayintersections cannot in some embodiments be fully determined in the TTU700 because TTU 700 performs the intersection test based on the geometryof the triangle and may not have access to the texture of the triangleand/or area of the triangle occupied by the texture (in otherembodiments, the TTU may be provided with texture information by thetexture mapping block of the graphics pipeline). To fully determinewhether the triangle is intersected, information about transparenttriangles the ray-primitive and transform block 710 determines areintersected may be sent to the SM 132, for the SM to make the fulldetermination as to whether the triangle affects visibility along theray.

The SM 132 can resolve whether or not the ray intersects a textureassociated with the transparent triangle and/or whether the ray will beblocked by the texture. The SM 132 may in some cases send a modifiedquery to the TTU 700 (e.g., shortening the ray if the ray is blocked bythe texture) based on this determination.

In one embodiment, the TTU 700 may be configured to return all trianglesdetermined to intersect the ray to the SM 132 for further processing.Because returning every triangle intersection to the SM 132 for furtherprocessing is costly in terms of interface and thread synchronization,the TTU 700 may be configured to hide triangles which are intersectedbut are provably capable of being hidden without a functional impact onthe resulting scene. For example, because the TTU 700 is provided withtriangle type information (e.g., whether a triangle is opaque ortransparent), the TTU 700 may use the triangle type information todetermine intersected triangles that are occluded along the ray byanother intersecting opaque triangle and which thus need not be includedin the results because they will not affect the visibility along theray. As discussed above with reference to FIGS. 5A-5C, if the TTU 700knows that a triangle is occluded along the ray by an opaque triangle,the occluded triangle can be hidden from the results without impact onvisualization of the resulting scene.

The intersection management block 722 may include a result queue forstoring hits that associate a triangle ID and information about thepoint where the ray hit the triangle. When a ray is determined tointersect an opaque triangle, the identity of the triangle and thedistance of the intersection from the ray origin can be stored in theresult queue. If the ray is determined to intersect another opaquetriangle, the other intersected opaque triangle can be omitted from theresult if the distance of the intersection from the ray origin isgreater than the distance of the intersected opaque triangle alreadystored in the result queue. If the distance of the intersection from theray origin is less than the distance of the intersected opaque trianglealready stored in the result queue, the other intersected opaquetriangle can replace the opaque triangle stored in the result queue.After all of the triangles of a query have been tested, the opaquetriangle information stored in the result queue and the intersectioninformation may be sent to the SM 132.

In some embodiments, once an opaque triangle intersection is identified,the intersection management block 722 may shorten the ray stored in theray management block 730 so that bounding volumes (which may includetriangles) behind the intersected opaque triangle (along the ray) willnot be identified as intersecting the ray.

The intersection management block 722 may store information aboutintersected transparent triangles in a separate queue. The storedinformation about intersected transparent triangles may be sent to theSM 132 for the SM to resolve whether or not the ray intersects a textureassociated with the triangle and/or whether the texture blocks the ray.The SM may return the results of this determination to the TTU 700and/or modify the query (e.g., shorten the ray if the ray is blocked bythe texture) based on this determination.

Example Ray Tracing Shading Pipeline

FIG. 10A shows an exemplary ray tracing shading pipeline 900 that may beperformed by SM 132 and accelerated by TTU 700. The ray tracing shadingpipeline 900 starts by an SM 132 invoking ray generation 910 and issuinga corresponding ray tracing request to the TTU 700. The ray tracingrequest identifies a single ray cast into the scene and asks the TTU 700to search for intersections with an acceleration data structure the SM132 also specifies. The TTU 700 traverses (FIG. 10A block 920) theacceleration data structure to determine intersections or potentialintersections between the ray and the volumetric subdivisions andassociated triangles the acceleration data structure represents.Potential intersections can be identified by finding bounding volumes inthe acceleration data structure that are intersected by the ray.Descendants of non-intersected bounding volumes need not be examined.

For triangles within intersected bounding volumes, the TTU 700ray-primitive test block 720 performs an intersection 930 process todetermine whether the ray intersects the primitives. The TTU 700 returnsintersection information to the SM 132, which may perform an “any hit”shading operation 940 in response to the intersection determination. Forexample, the SM 132 may perform (or have other hardware perform) atexture lookup for an intersected primitive and decide based on theappropriate texel's value how to shade a pixel visualizing the ray. TheSM 132 keeps track of such results since the TTU 700 may return multipleintersections with different geometry in the scene in arbitrary order.

Alternatively, primitives that the TTU 700 determines are intersectedmay be further processed to determine 950 whether they should be shadedas a miss 960 or as a closest hit 970. The SM 132 can for exampleinstruct the TTU 700 to report a closest hit in the specified geometry,or it may instruct the TTU to report all hits in the specified geometry.For example, it may be up to the SM 132 to implement a “miss” shadingoperation for a primitive the TTU 700 determines is intersected based onimplemented environment lookups (e.g., approximating the appearance of areflective surface by means of a precomputed texture image) such asshown in FIGS. 6A & 6B. The SM 132 may perform a closest hit shadingoperation to determine the closest intersected primitive based onmaterial evaluations and texture lookups in response to closest hitreports the TTU 700 provided for particular object geometry.

The FIG. 10B more detailed diagram of a ray-tracing pipeline flowchartshows the data flow and interaction between components for arepresentative use case: tracing rays against a scene containinggeometric primitives, with instance transformations handled in hardware.In one example non-limiting embodiment, the ray-tracing pipeline of FIG.10B is essentially software-defined (which in example embodiments meansit is determined by the SMs 132) but makes extensive use of hardwareacceleration by TTU 700. Key components include the SM 132 (and the restof the compute pipeline), the TTU 700 (which serves as a coprocessor toSM), and the L1 cache and downstream memory system, from which the TTUfetches BVH and triangle data.

The pipeline shown in FIG. 10B shows that bounding volume hierarchycreation 1002 can be performed ahead of time by a development system. Italso shows that ray creation and distribution 1004 are performed orcontrolled by the SM 132 or other software in the example embodiment, asis shading (which can include lighting and texturing). The examplepipeline includes a “top level” BVH tree traversal 1006, raytransformation 1014, “bottom level” BVH tree traversal 1018, and aray/triangle (or other primitive) intersection 1026 that are eachperformed by the TTU 700. These do not have to be performed in the ordershown, as handshaking between the TTU 700 and the SM 132 determines whatthe TTU 700 does and in what order.

The SM 132 presents one or more rays to the TTU 700 at a time. Each raythe SM 132 presents to the TTU 700 for traversal may include the ray'sgeometric parameters, traversal state, and the ray's ray flags, modeflags and ray operations information. In an example embodiment, a rayoperation (RayOp) provides or comprises an auxiliary arithmetic and/orlogical test to suppress, override, and/or allow storage of anintersection. The traversal stack may also be used by the SM 132 tocommunicate certain state information to the TTU 700 for use in thetraversal. A new ray query may be started with an explicit traversalstack. For some queries, however, a small number of stack initializersmay be provided for beginning the new query of a given type, such as,for example: traversal starting from a complet; intersection of a raywith a range of triangles; intersection of a ray with a range oftriangles, followed by traversal starting from a complet; vertex fetchfrom a triangle buffer for a given triangle, etc. In some embodiments,using stack initializers instead of explicit stack initializationimproves performance because stack initializers require fewer streamingprocessor registers and reduce the number of parameters that need to betransmitted from the streaming processor to the TTU.

In the example embodiment, a set of mode flags the SM 132 presents witheach query (e.g., ray) may at least partly control how the TTU 700 willprocess the query when the query intersects the bounding volume of aspecific type or intersects a primitive of a specific primitive type.The mode flags the SM 132 provides to the TTU 700 enable the ability bythe SM and/or the application to e.g., through a RayOp, specify anauxiliary arithmetic or logical test to suppress, override, or allowstorage of an intersection. The mode flags may for example enabletraversal behavior to be changed in accordance with such aspects as, forexample, a depth (or distance) associated with each bounding volumeand/or primitive, size of a bounding volume or primitive in relation toa distance from the origin or the ray, particular instances of anobject, etc. This capability can be used by applications to dynamicallyand/or selectively enable/disable sets of objects for intersectiontesting versus specific sets or groups of queries, for example, to allowfor different versions of models to be used when application statechanges (for example, when doors open or close) or to provide differentversions of a model which are selected as a function of the length ofthe ray to realize a form of geometric level of detail, or to allowspecific sets of objects from certain classes of rays to make somelayers visible or invisible in specific views.

In addition to the set of mode flags which may be specified separatelyfor the ray-complet intersection and for ray-primitive intersections,the ray data structure may specify other RayOp test related parameters,such as ray flags, ray parameters and a RayOp test. The ray flags can beused by the TTU 700 to control various aspects of traversal behavior,back-face culling, and handling of the various child node types, subjectto a pass/fail status of an optional RayOp test. RayOp tests addflexibility to the capabilities of the TTU 700, at the expense of somecomplexity. The TTU 700 reserves a “ray slot” for each active ray it isprocessing, and may store the ray flags, mode flags and/or the RayOpinformation in the corresponding ray slot buffer within the TTU duringtraversal.

In the example shown in FIG. 10B, the TTU 700 performs a top level treetraversal 1006 and a bottom level tree traversal 1018. In the exampleembodiment, the two level traversal of the BVH enables fast ray tracingresponses to dynamic scene changes.

Ray transformation 1014 provides the appropriate transition from the toplevel tree traversal 1006 to the bottom level tree traversal 1018 bytransforming the ray, which may be used in the top level traversal in afirst coordinate space (e.g., world space), to a different coordinatespace (e.g., object space) of the BVH of the bottom level traversal. Anexample BVH traversal technique using a two level traversal is describedin previous literature, see, e.g., Woop, “A Ray Tracing HardwareArchitecture for Dynamic Scenes”, Universitat des Saarlandes, 2004, butembodiments are not limited thereto.

In some embodiments, the top level traversal (in world space) is made ina BVH that may be dynamically recalculated (e.g., by SM 132) in responseto changes in the scene, and the bottom level traversal is made in a BVHof bounding volumes that remain static or substantially static even whenchanges in the scene occur. The bounding volumes in the BVH used for thebottom level tree traversal 1018 (in object space) may encompass moredetailed information regarding the scene geometry than the respectivebounding volumes used in the top level tree traversal 1006, therebyavoiding or at least reducing the modification of the bottom leveltraversal BVH in response to scene changes. This helps to speed up raytracing of dynamic scenes.

Example Top Level Tree Traversal

The top level tree traversal 1006 by TTU 700 receives complets from theL1 cache 1012, and provides an instance to the ray transformation 1014for transformation or a miss/end output 1013 to the SM 132 for closesthit shader 1015 processing by the SM (this block can also operaterecursively based on non-leaf nodes/no hit conditions). In the top leveltree traversal 1006, a next complet fetch step 1008 fetches the nextcomplet to be tested for ray intersection in step 1010 from the memoryand/or cache hierarchy and ray-bounding volume intersection testing isdone on the bounding volumes in the fetched complet.

As described above, an instance node connects one BVH to another BVHwhich is in a different coordinate system. When a child of theintersected bounding volume is an instance node, the ray transformation1014 is able to retrieve an appropriate transform matrix from the L1cache 1016. The TTU 700, using the appropriate transform matrix,transforms the ray to the coordinate system of the child BVH. U.S.patent application Ser. No. 14/697,480, which is already incorporated byreference, describes transformation nodes that connect a first set ofnodes in a tree to a second set of nodes where the first and second setsof nodes are in different coordinate systems. The instance nodes inexample embodiments may be similar to the transformation nodes in U.S.application Ser. No. 14/697,480. In an alternative, non-instancing modeof TTU 700 shown in FIG. 10C, the TTU does not execute a “bottom” leveltree traversal 1018 and noninstanced tree BVH traversals are performedby blocks 1008, 1010 e.g., using only one stack. The TTU 700 can switchbetween the FIG. 10B instanced operations and the FIG. 10C non-instancedoperations based on what it reads from the BVH and/or query type. Forexample, a specific query type may restrict the TTU to use just thenon-instanced operations. In such a query, any intersected instancenodes would be returned to the SM.

In some non-limiting embodiments, ray-bounding volume intersectiontesting in step 1010 is performed on each bounding volume in the fetchedcomplet before the next complet is fetched. Other embodiments may useother techniques, such as, for example, traversing the top leveltraversal BVH in a depth-first manner. U.S. Pat. No. 9,582,607, alreadyincorporated by reference, describes one or more complet structures andcontents that may be used in example embodiments. U.S. Pat. No.9,582,607 also describes an example traversal of complets.

When a bounding volume is determined to be intersected by the ray, thechild bounding volumes (or references to them) of the intersectedbounding volume are kept track of for subsequent testing forintersection with the ray and for traversal. In example embodiments, oneor more stack data structures is used for keeping track of childbounding volumes to be subsequently tested for intersection with theray. In some example embodiments, a traversal stack of a small size maybe used to keep track of complets to be traversed by operation of thetop level tree traversal 1006, and primitives to be tested forintersection, and a larger local stack data structure can be used tokeep track of the traversal state in the bottom level tree traversal1018.

Example Bottom Level Tree Traversal

In the bottom level tree traversal 1018, a next complet fetch step 1022fetches the next complet to be tested for ray intersection in step 1024from the memory and/or cache hierarchy 1020 and ray-bounding volumeintersection testing is done on the bounding volumes in the fetchedcomplet. The bottom level tree traversal, as noted above, may includecomplets with bounding volumes in a different coordinate system than thebounding volumes traversed in the upper level tree traversal. The bottomlevel tree traversal also receives complets from the L1 cache and canoperate recursively or iteratively within itself based onnon-leaf/no-hit conditions and also with the top level tree traversal1006 based on miss/end detection. Intersections of the ray with thebounding volumes in the lower level BVH may be determined with the raytransformed to the coordinate system of the lower level completretrieved. The leaf bounding volumes found to be intersected by the rayin the lower level tree traversal are then provided to the ray/triangleintersection 1026.

The leaf outputs of the bottom level tree traversal 1018 are provided tothe ray/triangle intersection 1026 (which has L0 cache access as well asability to retrieve triangles via the L1 cache 1028). The L0 complet andtriangle caches may be small read-only caches internal to the TTU 700.The ray/triangle intersection 1026 may also receive leaf outputs fromthe top level tree traversal 1006 when certain leaf nodes are reachedwithout traversing an instanced BVH.

After all the primitives in the primitive range have been processed, theIntersection Management Unit inspects the state of the result Queue andcrafts packets to send to the Stack Management Unit and/or RayManagement Unit to update the ray's attributes and traversal state, setup the ray's next traversal step, and/or return the ray to the SM 132(if necessary). If the result queue contains opaque or alphaintersections found during the processing of the primitive range thenthe Intersection Management Unit signals the parametric length (t) ofthe nearest opaque intersection in the result queue to the raymanagement unit to record as the ray's tmax to shorten the ray. Toupdate the traversal state to set up the ray's next traversal step theIntersection Management Unit signals to the Stack Management Unitwhether an opaque intersection from the primitive range is present inthe resultQueue, whether one or more alpha intersections are present inthe result queue, whether the resultQueue is full, whether additionalalpha intersections were found in the primitive range that have not beenreturned to the SM and which are not present in the resultQueue, and theindex of the next alpha primitive in the primitive range for the ray totest after the SM consumes the contents of the resultQueue (the index ofthe next primitive in the range after the alpha primitive with thehighest memory-order from the current primitive range in the resultqueue).

When the Stack Management Unit 740 receives the packet from IntersectionManagement Unit 722, the Stack Management Unit 740 inspects the packetto determine the next action required to complete the traversal step andstart the next one. If the packet from Intersection Management Unit 722indicates an opaque intersection has been found in the primitive rangeand the ray mode bits indicate the ray is to finish traversal once anyintersection has been found the Stack Management Unit 740 returns theray and its results queue to the SM with traversal state indicating thattraversal is complete (a done flag set and/or an empty top level andbottom level stack). If the packet from Intersection Management Unit 722indicates that there opaque or alpha intersection in the result queueand that there are remaining alpha intersections in the primitive rangenot present in the result queue that were encountered by the ray duringthe processing of the primitive range that have not already beenreturned to the SM, the Stack Management Unit 740 returns the ray andthe result queue to the SM with traversal state modified to set the cullopaque bit to prevent further processing of opaque primitives in theprimitive range and the primitive range starting index advanced to thefirst alpha primitive after the highest alpha primitive intersectionfrom the primitive range returned to the SM in the ray's result queue.If the packet from Intersection Management Unit 722 indicates that noopaque or alpha intersections were found when the ray processed theprimitive range the Stack Management Unit 740 pops the top of stackentry (corresponding to the finished primitive range) off the activetraversal stack. If the packet from Stack Management Unit 740 indicatesor that either there are opaque intersections in the result queue andthe ray mode bits do not indicate that the ray is to finish traversalonce any intersection has been found and/or there are alphaintersections in the result queue, but there were no remaining alphaintersections found in the primitive range not present in the resultqueue that have not already been returned to the SM the Stack ManagementUnit 740 pops the top of stack entry (corresponding to the finishedprimitive range) off the active traversal stack and modifies thecontents of the result queue to indicate that all intersections presentin the result queue come from a primitive range whose processing wascompleted.

If the active stack is the bottom stack, and the bottom stack is emptythe Stack Management Unit 740 sets the active stack to the top stack. Ifthe top stack is the active stack, and the active stack is empty, thenthe Stack Management Unit 740 returns the ray and its result queue tothe SM with traversal state indicating that traversal is complete (adone flag set and/or an empty top level and bottom level stack). If theactive stack contains one or more stack entries, then the StackManagement Unit 740 inspects the top stack entry and starts the nexttraversal step. Testing of primitive and/or primitive ranges forintersections with a ray and returning results to the SM 132 aredescribed in U.S. application Ser. No. 16/101,148 entitled “ConservativeWatertight Ray Triangle Intersection”, U.S. application Ser. No.16/101,066 entitled “Method for Continued Bounding Volume HierarchyTraversal on Intersection without Shader Intervention” and U.S.application Ser. No. 16/101,196 entitled “Method for HandlingOut-of-Order Opaque and Alpha Ray/Primitive Intersections”, which arehereby incorporated by reference in their entireties.

While the above disclosure is framed in the specific context of computergraphics and visualization, ray tracing and the disclosed traversalcoprocessor could be used for a variety of applications beyond graphicsand visualization. Non-limiting examples include sound propagation forrealistic sound synthesis, simulation of sonar systems, design ofoptical elements and systems, particle transport simulation (e.g., formedical physics or experimental high-energy physics), general wavepropagation simulation, comparison to LIDAR data for purposes e.g., ofrobot or vehicle localization, and others. OptiX™ has already been usedfor some of these application areas in the past.

Example Implementation Traversal Coprocessor Provides Structure toInterrupt and Resume Traversal Operations

As described above, the Tree Traversal Unit (TTU) 700 is a ray tracinghardware accelerator closely coupled to the SM 132. The TTU 700 operatesby tracing rays through a generated Bounding Volume Hierarchy (BVH)which includes internal nodes, instance transforms, item ranges, andtriangle ranges. Internal node intersections traverse further into thehierarchy. Instance node intersections can do a hardware-acceleratedinstance transform and then continue traversal on the instanced childBVH. Item ranges are returned to the SM 132. Triangle ranges can usespecific hardware acceleration inside the TTU 700 in the Ray-PrimitiveTest (RPT) unit to determine an intersection between a ray and aspecific primitive such as a triangle.

The SM 132 has a mechanism for preemption that allows for a warp orwarps (e.g., threads) currently running on the SM to be paused with theintent to swap them out for other work. This is a means for time-slicesharing of the SM 132, as well as a mechanism to respond to and preventhangs from poorly written programs.

When the SM 132 receives or generates a preemption signal, thatnotification is passed as well to the TTU 700. In other words, the SM132 can itself decide to preempt the TTU 700's current tasks, or the GPUcan decide to preempt the SM's current tasks, or the SM can decide toswap out work. In any case, the TTU 700 receives the preemption signal.The TTU 700 will then cleanly stop execution for all active rays andreturn to the SM 132 any intermediate and/or final results along withstate information which when provided later to the TTU will allow theTTU to restart the query exactly where it left off if traversal is notcomplete.

Each ray in the TTU 700 is operated on independently based on its owntraversal requirements. In the case of an intersection that requires SM132 intervention (e.g., item ranges), the TTU 700 returns to the SM 132an opaque (to the SM) blob of data which includes the stack entries andmeta information that allow a query to continue exactly where it leftoff. The SM 132 does not do anything with this opaque blob of data otherthan store it temporarily for safekeeping and provide it back to the TTU700 if/when the SM 132 wants the TTU 700 to resume work on that query.In example non-limiting implementations, the SM 132 keeps the opaqueblob for the TTU 700 because the TTU 700 is stateless—it can do thingsvery fast but can't remember anything about any particular job it hasdone before.

The TTU 700 is a little like a speedy and highly efficient robot whoperforms tasks very quickly but can't remember which particular tasks heor she worked on before. In the example non-limiting implementations,the TTU 700 can partially complete a job and return results back to theSM 132 so the SM can decide whether to have the TTU continue to work onthat job—and if so, whether to perform particular aspects, subsets orexpansions of the job.

Part of the contract with software is that the TTU query is able to beresumed after preemption. For example, typically there would be an outerloop around the sequence—but in the case of the TTU 700 there must be(in example non-limiting implementations) an outer loop around thesequence. Even if there are no alpha triangles, custom primitives or thelike and even if the geometry consists of things the TTU 700 knows howto handle, because of the possibility of preemption there is still achance that the TTU 700's traversal will not complete. Thus, even thoughthe TTU 700 returns results to the SM 132, there may still be more workto do in a followup query from the SM to the TTU. For at least thisreason, any query the SM 132 makes to the TTU 700 may take or involvemultiple requests for completion to occur.

In the example non-limiting implementation, the TTU 700 does this bysending a signal to the Stack Management Unit (SMU) 740 which isresponsible for activation of the next traversal step after the currentone completes. If the preemption signal is set, the TTU 700 will notactivate the next traversal step, but will cause a return to the SM 132.In this way, each ray can finish the traversal step it is currentlyinvolved in—hence a “clean stop”. Once all rays have been halted andresults returned to SM 132, the preemption completes and thecorresponding warps are swapped out in the SM.

The Streaming Processor Cannot Trap Until All Dependent Operations HaveCompleted

The example non-limiting implementation provides an SM 132 with amechanism that can selectively stop warps or threads from running,causing preemption so those warps can be swapped out for other work tocome in. This is thus a way to time-slice the SM 132. Meanwhile, inexample non-limiting implementations, the SM 132 is prevented fromtrapping or being preempted as long as there is outstanding work thatother parts of the GPU owe the SM. Texture mapping and otherhardware-accelerated processes can be counted on to return resultswithin a short amount of time. For example, the longest latency the SM132 typically will see is on a memory load. Most other uncompletedprocesses the SM 132 may be waiting on to complete before trappinggenerally have fixed and short latencies.

But the TTU 700 is a challenge for the SM 132 to preempt because anygiven query the SM sends to the TTU could take an unpredictable lengthof time such as thousands or even hundreds of thousands of cycles tocomplete, depending on a variety of factors such as the BVH beingtraversed, the spatial relationship between the ray the query specifies,the kind of geometry (e.g., alpha as opposed to opaque) the rayintersects, how many memory loads are required, whether the data isalready in cache memory (and if so which one), other internal schedulingissues within the TTU 700, etc. The fact that the TTU 700 canpotentially take a comparatively long, variable amount of time tocomplete any given query creates a risk that the SM 132's preemptionmechanism will be rendered useless in certain cases in which the TTU 700takes a long time to complete a query, thereby preventing the SM fromever trapping. In contrast, it would be desirable to allow the SM 132 toforce a ray operation preemption onto the TTU 700 while ensuring thatthe TTU will preempt gracefully, in a predictable manner and within areasonable (low latency) amount of time.

In more detail, the example non-limiting processes may use “scoreboards”to track dependencies on outstanding TTU 700 loads or reads. In examplenon-limiting implementations, the SM 132 cannot be preempted until thatdata comes back to the SM from the TTU 700—which generally won't happenuntil traversal completes. As long as a scoreboard is outstanding, theSM 132 is not permitted to enter the trap handler, so it sits therewaiting. Worse case is preemption might be delayed for hundreds ofthousands of cycles, or even longer if there is a badly structured BVHthat contains an internal (e.g., endless) loop (for example, a pointerwithin the BVH tree points to the root of the tree to cause traversal ofthe tree to be endless, meaning that the graph is no longer a tree).This could happen due to a bug or could constitute a malevolent attackon the system in an attempt to tie up the system indefinitely. On ashared machine, such a loop could permit an attacker to implement adenial-of-service attack unless some mechanism is provided to interruptthe traversal.

One potential solution to such a problem is to use a watchdog timer tokill TTU 700's tasks that take too long to complete. While this approachwould serve to protect the system, it is not very flexible and does notprovide for graceful recovery for the software running on SM 132.

Preemption Management

In the example non-limiting implementations, an SM 132's request orquery to the TTU 700 is something the TTU can return back to the SMwhile in progress. For example, if the TTU 700 encounters an alphatriangle while traversing the BVH on behalf of a ray, the TTU can returnquery results in mid-traversal. See copending commonly-related USapplication Ser. Nos.: 16/101,066: titled “Method for Continued BoundingVolume Hierarchy Traversal on Intersection without Shader Intervention”;16/101,180 titled “Query-Specific Behavioral Modification of TreeTraversal”; and 16/101,196 titled “Method for Handling Out-of-OrderOpaque and Alpha Ray/Primitive Intersections”. A query can thus involvemultiple requests between an SM 132 and the TTU 700; and a mechanismexists to allow the TTU to return intermediate traversal state to theSM. The example non-limiting implementation takes advantage of thisability to naturally interrupt traversal and to later resume in order toprovide further functionality—namely preemption management. In suchpreemption management, the SM 132 can preempt the entire TTU 700,stopping all work the TTU is doing. This means that all ray processingthe TTU 700 is performing should stop. The SM 132 can order preemptionfor any number of reasons, including for example the need to reallocatethe TTU 700 to temporarily do more other, important work (after whichthe SM can order the TTU to resume where it left off).

The Example Non-Limiting Implementation Provides a Forward ProgressGuarantee

Imagine a carpenter trying to do a job at a jobsite. If it takes thecarpenter 30 minutes at the beginning of the work day to set up histools, sawhorses, extension cords, supplies, etc., before he beginswork, and 30 minutes at the end of the work day to clean up and put awayall of the tools, then interrupting the carpenter and sending him to adifferent job site after 25 minutes on the job means he will get no workdone at all. The carpenter will not even have finished unpacking andsetting up his tools before he needs to start packing them up again.

Similarly, because of overheads in starting a TTU 700 query and ingetting the first data for traversal, it is possible that a series ofpreemptions could cause competing work to not make forward progress asthe competing work tasks continually preempt each other without either(or any) of them making forward progress.

To avoid that issue, the TTU 700 in example non-limiting implementationshas a forward progress guarantee: each ray that starts a query isguaranteed to make at least one traversal step that modifies thetraversal stack content. In the example non-limiting implementation, theTTU 700 will thus put off the preemption command for a ray until it hascompleted at least one traversal step (which in the example non-limitingimplementation can be measured by a stack push or a stack pop). Toimplement the forward progress guarantee, the initial activation for aray is ignored when there is a preemption signal and instead ofreturning to SM 132, it will activate the correct data path as directedby its stack content.

A traversal step can also be measured with a stack entry modification.For example, an alpha triangle intersection can modify the trianglerange on the stack and that can be counted as a traversal step. In thatcase though, the alpha triangle is returned to the SM 132 at that pointanyway and so its inclusion might be somewhat moot. See below.

In the example non-limiting implementation, the TTU 700's expectedbehavior is that it will complete some meaningful work even if the SM132 tries to preempt it immediately after it receives a query command.Thus, to avoid a scenario where the TTU 700 makes no forward progress,the example non-limiting implementations provide a forward progressguarantee that if a ray starts a ray traversal in the TTU 700, it getsto make at least one stack-modifying action before it is preempted.

FIGS. 11A-11H show an analogy. In FIG. 11A, a robot representing the TTU700 is doing some work, i.e., a ray processing traversal step. When therobot has completed the step, the robot places the results on a stack(FIG. 11B) and starts working on the next traversal step (FIG. 11C).When the robot completes the next step, the robot places those resultson the stack (FIG. 11D) and starts working on the next traversaloperation (FIG. 11E). But FIG. 11E shows that the robot receives acommand (from SM 132—the “boss”) to stop work. Instead of obeying thepreemption command immediately, the robot (because he knows he issupposed to make forward progress) completes the current traversal stepFIG. 11F) and places the results on the stack (FIG. 11G). By completingthis traversal step, the robot guarantees forward progress (FIG. 12block 3002) no matter how often the robot is interrupted.

Once the robot completes the traversal step and places the results onthe stack, the robot ceases work (FIG. 11H) and sends the stack to theSM 132 for safekeeping. The robot then waits for further instructions(in some implementations, the robot can immediately begin doingdifferent work instead of just waiting).

Similarly, in example non-limiting implementations, when the SM 132wants to preempt, it sends a preemption signal to the TTU 700 (see FIG.12). If the TTU 700 were simply to stop processing immediately uponreceiving the SM 132's preemption signal, there could be a scenariowhere the TTU never makes forward progress. There is a finite amount ofoverhead time involved in setting up a TTU 700 query and getting resultsback to the SM 132. One might think it would be desirable to preemptfaster than that overhead. So suppose the SM 132 sets up a query withthe TTU 700, then soon thereafter sends the TTU a preemption signal thatforces the TTU to stop doing whatever it is doing. The TTU 700 wouldstop in response to the preemption signal and send its results back tothe SM 132. If such preemption were to happen repeatedly in rapidsuccession, the TTU 700 would not be making forward progress. Rather,like the carpenter described above, the TTU 700 would be spending all ofits time setting up and breaking down the successive queries, and wouldnot be spending any of its time actually performing a query.

To save area, the example non-limiting implementation does not directlytrack or attempt to directly track whether a ray has in fact madeforward progress since the last preemption. When the TTU 700 receivesthe preemption signal, the TTU makes sure any (each) ray that is activemakes one more step. Generally, by observing since the time thepreemption signal is asserted that each active ray has made forwardprogress, the system can guarantee that all unfinished rays will havemade some forward progress. It might be an unusual case for the TTU tomake no forward progress due to repeated preemptions, but awell-designed robust system should be able to handle even such unusualoccurrences without failing.

In example non-limiting implementations, forward progress (see FIG. 12block 3002) means the TTU 700 can or will push something onto thetraversal stack, pop something from the stack or modify an entry alreadyon the stack. In example non-limiting implementations, the TTU 700 isnot considered to have made forward progress unless and until itperforms or will perform at least one of these stack-modifying actions.Meanwhile, the SM 132 will delay preemption until the TTU 700 returnsstatus and results to the SM after having performed (or being guaranteedto perform) such a stack-modifying action constituting forward progress(see FIG. 12, “no” exit to block 3002).

Stack-Based Forward Progress Guarantee

In example non-limiting implementations, a short-stack mechanism withinSMU 740 tells the TTU 700 what work needs to be done next. The stackwill have entries that are, for example, complet traversals that areused to test the ray against bounding boxes in the BVH. The stack willalso have entries that specify geometry (e.g., triangle) ranges the TTU700 is to test the ray against. The short stack will also containentries that indicate instance nodes that the TTU 700 needs to perform atransform against. The stack may also include item ranges or nodereferences that need to be returned to the SM 132.

In the example non-limiting implementation, the complet test entrieswill push entries onto the stack as the TTU 700 traverses deeper intothe BVH data structure and pop entries from the stack as the TTUtraverses upward in the BVH. The triangle range portion in which the TTU700 tests the ray against triangles will modify stack entries (as theTTU performs intersection tests against indicated particular trianglesin the range) or, if the TTU has performed intersection tests againstall triangles in the range, the TTU may pop the completed entry from thestack. An instance node that's on the top (world space) stack (i.e.,that is represented in world space) will cause the TTU 700 to perform atransform into object space, after which the instance node will cause anentry (i.e., the root of the instanced BVH) to be pushed onto the bottom(object space) stack so the TTU can continue traversal (e.g., in objectspace of the subtree of the instance node BVH) on the bottom stack whileleaving the top stack as it is. The bottom stack now will have contentsthat needs to be initialized (or that may be initialized by thetransform) that will then be executed by the TTU 700. The bottom stackwill generally have complets that need to be tested against the raybased on any number of traversals of the BVH, and will also containtriangle range entries as the traversal proceeds. The bottom stack mayalso have instance nodes and item ranges that need to be returned to theSM 132.

While instance nodes in world space will continue traversal, in theexample non-limiting implementation the mere switching to object spaceis not enough to demonstrate forward progress. The example non-limitingimplementation uses, as indicia of forward progress, a modification ofthe stack issued to demonstrate for example that the TTU 700 has:

done something in the ray-complet test (i.e., pushed a complet onto thestack for a downward traversal of the BVH, or a pop of the complet justexamined off of the stack) (See FIG. 12A block 3012); or

in the case of a triangle intersection test, pop of a triangle rangefrom the stack or modified the triangle range represented by a stackentry in the case for example of alpha triangles where the TTU shortensthe ray and/or otherwise limits the triangle range to a subset of thefull range (see FIG. 12A, block 3014). In one implementation, the TTU700 can limit the triangle range without modifying the ray. For example,suppose an alpha triangle returns to the SM 132. If the SM determines ahit, the SM can relaunch the query with a modified t-range for the ray.Before that, the TTU 700 modifies the triangle range stack entryinternally to point to the next triangle to test on return.

Because of the way the SM 132 specifies rays in the query setup suchthat they are always in the world space, the first thing the examplenon-limiting TTU 700 needs to do if it is resuming a previous query inthe object space is to again transform the ray from world space toobject space. Since the TTU 700 is stateless and always starts traversalwith the top or world space stack, for an instanced traversal, the firststep always done is an instance transform. This step does not modify thetraversal stack content for that instance node entry, but rather shiftsexecution from the top or world space stack to the bottom or objectspace stack. (The transform may push entries onto the bottom stack.)Thus, in example non-limiting implementations, instance nodes merelymove the TTU processing from the top (world space) stack to the bottom(object space) stack without changing the entry on the top stack andwithout yet pushing an entry onto the bottom stack.

If that is the only action the TTU 700 has yet performed since resumingthe query, when the query is relaunched that TTU will need to performthat same action again. Since the transform node does not modify theinstance node stack entry, an instance transform may be done andrequired any number of times for the same traversal. If that were to bethe only step allowed in the forward progress guarantee, then it wouldbe insufficient for forward progress since that step does not modify thetraversal stack and would simply be repeated again and again when thequery is relaunched. For this reason, in the example non-limitingimplementation, an instance modifier is not considered a stack modifierfor the purpose of demonstrating forward progress (see FIG. 12A, block3014, “NO” exit and branch to block 3012 after continuing). Because ofthat, the activation after an instance transform (like the initialactivation) is also disallowed for redirection to SM 132 on preemption.In that way, the bottom or object-space stack is inspected and at leastone traversal step that modifies that stack is completed beforepreemption can occur.

It is also possible that a ray needs to perform more than one traversalstep. In the example non-limiting implementation, there are certainstack entries which are called auto-pop entries. In example non-limitingimplementations, an auto-pop entry is typically limited to an opaquetriangle range to be processed in the TTU 700. The existence of theseentries is because a user can specify a reduced stack limit thatrestricts what can be returned to the SM 132. For performance reasons,we often want to internally exceed that stack limit and that can besafely done for those stack entries which cannot leave an entry on thestack or push additional entries onto the stack. An opaque trianglerange is guaranteed to pop itself from the stack and does not generatenew entries to the stack. A ray that is preempted with opaque triangleranges on the top of the stack may need to consume each of those entriesto fit within the stack limit for return to the SM 132. For simplicityalone, some non-limiting implementation designs assume that all opaquetriangle ranges are auto-pop for preemption reasons—although that is nota functional requirement for other implementations.

Within the TTU 700 in example non-limiting implementations, the stackmanagement unit (SMU) 740 has control circuitry that looks at the topstack entry and decides what the TTU will do next. For example,depending on the contents of the top stack entry, the SMU 740 may sendthe corresponding work down the ray-complet text 710 path or insteaddown the ray-triangle test 720 path. Sending such work down one of thesepaths means the TTU 700 is taking another step in the current query.Alternatively, the SMU 740 can signal to the interface from the queryingSM 132 that the current ray is done. The SMU 740 also has a record ofthe immediately previous processing—for example, whether the last testwas a ray-complet test or a ray-triangle test. The SMU 740 knows forexample that if the last operation was a ray-complet test, then atraversal step has in fact been taken, forward progress has been madeand the next operation will be to pop an entry off of the stack. If thelast operation was a ray-triangle test based on a triangle range, thenthe SMU 740 knows it will modify the corresponding stack entry. If thelast operation was an instance transform, then the SMU 740 knows it willnot be modifying the stack such that no forward progress will bedeclared. What the SMU 740 is testing in these examples is that everyray got through an activation or was finished.

A fourth possible condition is initial ray activation, which comes fromthe ray management unit 730. In the example non-limiting implementation,the ray management unit (RMU) 730 declares that a new ray has beenactivated, and directs the SMU 740 to look at the top stack. Asdiscussed above, this is not enough for forward progress in examplenon-limiting implementations because no stack changes have been made.

Since the SMU 740 knows where the processing has come from (i.e., thelast step that was performed), the SMU can determine whether forwardprogress has been made. It is possible that since the last preemption orother event, the TTU 700 has made multiple steps of traversal. Thiscould be saved in one or more flags. In the example non-limitingimplementation, in order to save chip area, the TTU 700 does not recordthat fact. The impact is that the example non-limiting implementationmay take an extra step of traversal when such an extra step was notstrictly necessary to provide a forward progress guarantee. A singletraversal step is typically not a long latency event. In other examplenon-limiting implementations, the type of stack update that has or willoccur could be used as a forward progress guarantee (e.g., to ensurethat the stack will be updated to reflect that a leaf node in the BVHhas been reached or processed).

The example non-limiting implementation mechanism for guaranteeingforward progress does not need to use the stack modification itself as atrigger, but instead uses “where you have come from” information thatimplies a stack modification. The forward progress guarantee mechanismin example implementations thus only needs to know that the stack willbe modified. It does not need to actually observe the stack modificationto prove it occurs, nor does it need to know exactly how the stack wasor will be modified. The SMU 740 in making the forward progressdetermination examines previous state of the TTU 700, which implies thatthe stack will in fact be modified. The stack is a kind of a checkpointthat represents where a particular ray is in the course of traversal ofa BVH and how it got there. The concept of using a stack update as anindicator of forward progress works well because the stack indicates thetrue current state of the TTU 700, so using an update to that stack is areliable mechanism to indicate forward progress.

In the example non-limiting implementation, TTU 700 allows SM 132 toaccess and save off the TTU's state. In the example non-limitingimplementation, the TTU 700 is stateless, meaning that once itterminates a particular process it entirely “forgets” what it did andhow far it got. This feature allows a high degree of scaleability anddoes not require the TTU 700 to have write access to main memory. When atrap occurs and the TTU 700 has confirmed forward progress for eachactive uncompleted ray it is currently handling, the SM 132 saves offthe TTU 700's state for that thread or execution slot, i.e., the resultof the TTU's traversal so far and sufficient additional information forthe TTU to recover its state and continue where it left off (to do this,the SM does not need to know how far the TTU got in the process, sincethe SM is saving the state of the TTU for the execution slot/threadbeing preempted).

The TTU 700 returns to the SM 132 whatever hit (or hits) are currentlystored. In most preemption cases, these are intermediate hits that needto be stored in some manner (either in registers or spilled out tomemory). An intermediate hit may or may not be the same as the finalhit. The TTU 700 will continue traversal based on the stack and rayinformation that is returned. In the example embodiment, it is up to theSM 132 in the case of intermediate hits to also shorten the ray tocorrespond to the returned hit (rather than passing back in the originaltmax).

From an SM 132 perspective, there is no difference between a TTU 700return due to preemption and a TTU return due to the TTU needing SMassistance (e.g., alpha rays, item ranges, etc.). An exception is thatin case of preemption, the TTU 700 can return a miss result paired witha stack that requires further traversal (in the example embodiment, theTTU would normally complete traversing before reporting a miss). In theexample non-limiting embodiment, the SM 132 trap handler is responsiblefor storing the register contents for that thread in a general sense.Those registers will contain the information returned by TTU 700, butalso any other information the SM has on hand. When the threads arerestored, the SM 132 trap handler is responsible for restoring thecontent of those registers. The SM 132 then inspects the TTU return andacts upon it just as if the trap hadn't happened. Namely, if the resultis only an intermediate one, the SM 132 stores that result off andrelaunches the query so the TTU 700 can continue where it left off.

Example Non-Limiting Programmable Timeouts

We build upon that preemption timeout and forward progress guarantee toimplement user programmable timeouts. We detail two variants: work-basedand cycle-based. Such timeouts can be used to automatically discontinueprocessing for a particular ray. In example non-limitingimplementations, a TTU-wide timeout is reserved for preemption and isnot triggered by individual timeouts for individual rays.

It is possible to provide user-programmable timeouts that build uponthis same mechanism. Graphics developers are accustomed to modelingscenes and controlling graphics so they can guarantee that frames willbe processed within a frame time such as 16 ms (corresponding to 60 Hz).With ray tracing, this may not be the case. As discussed above, it maytake the TTU 700 an unpredictably long amount of time or cycles toprocess any given ray. For example, a given ray may happen to skim by alot of geometry (e.g., picture a ray that runs parallel to a bridge oralong a chain linked fence), such that it is to be tested against largenumbers of BVH nodes to determine intersection. It may be useful toprovide controls to allow developers to limit the amount of time,processing cycles or both for processing a given ray.

In the example non-limiting implementation, a timer is associated withhow long it takes the TTU 700 to process a given ray, not how long ittakes to process all rays in the scene or even all rays in a given warpor thread.

For example, in example implementations, rather than timing out anentire TTU 700, it is possible to timeout a single ray being processedby the TTU. Such a mechanism can be used to control the amount of workthat single ray does. For example, it is possible to put an upper boundon how long processing of a particular ray is going to take. This isparticularly useful in virtual reality, which requires a result in acertain amount of time to reduce display latency and avoid VR sickness(which can feel like motion sickness). Since a ray traversal may taketoo long in an arbitrary case, it may be desirable to put a bound on thelength of time the ray traversal will take. This could provide a way togive up on a ray that the system shot into the scene and (because thattask is taking too long) fall back on another mechanism to draw theimage in a shorter amount of time.

As shown in FIG. 13, one example implementation of a work-based timeoutprovides a counter per ray and a programmable target per ray. Every timethe TTU 700 performs an action (traversal step) such as a ray-complettest or a ray-triangle test with respect to a particular ray, the TTUincrements a corresponding counter 3200 for that ray. The counter couldcount each traversal step no matter what type, or it could count leafnode traversal steps, or there could be separate counters for differentkinds of traversal steps. Once that counter(s) 3200 exceeds theprogrammable target 3202 for that ray, the TTU 700 stops the ray andsends the results back to the SM 132.

Because this is a clean stop, if the SM 132 calling kernel wanted to, itcould restart the ray and have the TTU 700 finish the work. Or the SM132 could abandon the ray and use a different approach to finishprocessing the current image. Because this technique is applied per ray,each ray can be programmed to have its own target value which whenexceeded will trigger a cease execution. Different rays can takedifferent amounts of traversal steps before the TTU 700 stops them fromproceeding and asks the SM 132 what to do next depending on what else isgoing on.

The work-based timeout provides a means for a user to restrict per raythe number of traversal steps allowed. In one example embodiment, thework-based timeout stores a single value, which saves area. This valueis initialized to the target value, decremented on each traversal step,and triggers per-ray timeout when it reaches 0. Triggering couldalternatively occur on the transition from 1 to 0 so the 0 value can bereserved as a special case to disable timeouts. In that case, anauto-pop entry on the top of the stack after the 1-to-0 transition wouldincrement the counter back to 1 so as to still see the 1-to-0 transitionafter the auto-pop entry is processed.

In an alternative embodiment, the work-based timeout can be based on twovalues that are compared to each other. In this two-value embodiment,during the query setup for a ray, an additional N-bit field is includedwhich specifies a target number of traversal steps. That target isstored in SMU 740 alongside a counter of the traversal steps for thatray. The traversal step counter is incremented on every activation afterthe initial activation (i.e., any activation occurring after aRay-Complet Test or Ray-Triangle Test data path event). A comparatorcontinually compares the (incremented) traversal step counter with thetarget number of traversal steps. When (and if) the traversal countexceeds the target, a timeout bit which exists per-ray is set. When thatbit is set, the next activation will cause that individual ray totimeout just as it would if the entire TTU 700 were preempted. Otherrays are not affected by that action.

In some implementations, the number of traversal steps performed (or thenumber of steps remaining, in the case that the SMU stores only acounter that counts down to 0) could be returned to SM 132 as part ofthe result of the query. This would allow the developer to limit thetotal number of steps performed for a ray across multiple queries (e.g.,when alpha triangles or primitives that the TTU cannot handle nativelyare involved). For example, let's say that the developer initiallylimited the total number of steps to 100, and the TTU 700 returned aresult to SM 132 after only 20 steps for an alpha test. If the alphatest indicates a miss, then it may be desirable to limit the subsequentquery (continuation of the traversal) to only 80 steps. A similar schemecould be applied to cycle-based timeouts (below).

Programmable timeouts are not necessarily required in examplenon-limiting implementations to adhere to the no-timeout-after-transformrequirement for the preemption forward progress guarantee. In the casethat an example non-limiting implementation chooses not to adhere tothat, it is up to the user to specify at least 2 traversal steps or torecognize the case and specify the 2 traversal steps on relaunch.

Additionally, just as in the preemption case, it is possible that theactual number of traversal steps exceeds the targeted number oftraversal steps because of auto-pop entries.

The number of bits used to encode the work target need not be any numberin specific. It also need not be at a granularity of 1 entry. That is,the N bits could be shifted or multiplied so that they can target morethan just the number that can be specified by N. For example, 8 bitsallow for a target between 0 and 255, where 0 could be “disabled”. Byallowing for a shift of those bits, it could be target a highergranularity such as 4 to 1023 in increments of 4. Or 8 to 2048 inincrements of 8. That shift amount can also be made programmable perray. If the shift is programmable per-ray, then the per-ray targetsstored in SMU 740 accounts for all possible bits necessary, rather thanjust the N bits specified. The per-ray traversal counters in SMU 740 mayhave a counting granularity of 1.

Example Non-Limiting Cycle-Based Timeouts

The cycle-based timeouts work in a similar manner to the work-basedtimeouts but, as the name implies, are time based. Global counters (seeFIG. 13A, block 3302, 3304) in SMU 740 keep track of “epochs” which aredefined as some number of cycles, e.g., 1024. A counter 3302 in SMU 740advances the cycle count until the epoch count is hit, and then theepoch counter 3304 advances by 1. The epoch counter 3304 roughlydetermines the current time to within that granularity. It is possibleto have an epoch counter 3304 where the epoch is defined as a singlecycle. In that case, there is only a single counter, and the two-levelcounter design is not needed.

A cycle-based timeout for a ray is programmed in terms of epochs. Duringquery setup when writing into SMU 740, each ray adds the current epochcounter value to its targeted number of epochs, allowing for wrapping,and stores that value per ray in a target register 3306.

When the per-ray targeted counter matches the epoch counter (i.e., theepoch counter has advanced to the timeout point) (comparators 3308), theray is set to terminate at the next step. Operations after that pointare identical to operations for the work-based timeouts previouslydescribed.

The cycle-based timeout is only accurate to within the granularity of anepoch plus the time to finish its last traversal steps, which couldinclude some number of long latency memory accesses as well as anynumber of auto-pop steps. In this way, the cycle-based timeout is lessprecise than the work-based timeouts in terms of hitting a target, butstill effectively limits traversal to a desired time frame.

This cycle-based timeout thus operates by programming the maximum lengthof time the ray is allowed to complete instead of programming an exactnumber of traversal steps. The length of time could be expressed interms of the number of TTU cycles for example. Upon activating the ray,the TTU 700 would start a timer 3302 that counts elapsed TTU cycles.Once the timer exceeds an SM-specified target number of cycles, the TTU700 stops the ray. Because this is a clean stop, the calling SM 132 cancontinue the traversal if it chooses to, or it may abandon the raybecause it is taking too long to process and fall back to some othermechanism.

In one example implementation, rather than having a long (e.g., a64-bit) counter per ray, a single counter 3302 could be shared among allthe rays. A continual cycle counter counts cycles that have elapsed.Each time the cycle counter counts a certain number of cycles (an“epoch”) such as 1024 cycles, an epoch counter 3304 increments by one.The epoch counter thus counts epochs. The SM 132 sends to the TTU 700,for each ray, the number of epochs the SM will allow that ray before theray will be stopped due to timeout. For example, if the SM programs theray time value to be “5”, this will provide between 5×1024 cycles (five“epochs”) and 6×1024 cycles (6 epochs) for traversal before a timeout(in this example, it is possible to start a ray anytime during anepoch). Note that a clean stop can add on further time. The cycle-basedtimeout thus is not quite as accurate as a work-based timeout but may besufficiently accurate for many usages.

For example, there is a use case in which SM 132 has requested the TTU700 to process 32 rays, and some (for example 28) rays are done withsome stragglers not yet finished processing. If those stragglers areprogrammed to timeout earlier than full completion, then all ray resultsin that request can be returned earlier. In that case, the rays that arefinished free up processing capacity in the TTU 700, and the SM 132 asksthe TTU to resume processing of the rays that have not yet completedwhile at the same time sending more (new) rays for the TTU to beginprocessing. This provides higher TTU 700 utilization and increasedefficiency. This example shows that the TTU 700 can be busy doing workon other rays while it is waiting for straggler rays to completeprocessing. Thus, while the SM 132 has the option of simply abandoningrays that are taking too long, the SM has another option of packagingall of those straggler rays together and sending them back to the TTU700 for completion. By grouping these straggler rays together, it islikely that processing will be more efficient than if the SM 132 waitsfor all the stragglers to finish in the first pass.

The work-based and cycle-based timeouts allow for Quality-of-Servicetype guarantees which can be useful for any application which requireshard limits on work. The additional programmable timeouts can helpenable timing guarantees for applications like ray tracing in virtualreality (VR) applications and provide a deterministic debug mechanism.For example, Virtual Reality (VR) requires consistent frame rates andrefresh or the user experience degrades and can eventually cause userdiscomfort. It may be preferable to draw a wrong image than to reducethe refresh rate and cause users to experience vertigo or be nauseated.Programmable timeouts can also allow for single-step debug modes usefulfor Silicon bring up and internal debug. The preemption mechanism andforward progress guarantee is a helpful mechanism for the TTU 700 usedfor ray tracing in DirectX Raytracing (DXR) and OptiX™.

Other Example Implementations

In other implementations, the TTU 700 sets a bit or flag each time itmakes forward progress on a particular ray. This would avoid the need totake an extra step to guarantee forward progress has been made, at theexpense of additional area and circuit complexity associated withreading and writing the additional bit or flag.

A stack is helpful but not necessary to implement the examplenon-limiting implementations. For example, there are stackless traversalmethods that can be used to traverse a BVH. In fact, the examplenon-limiting TTU 700 implementation shown in FIG. 9 is kind of a hybridbetween a stackless traversal architecture and a traditional approachthat uses a stack that can grow without bounds. In one example, the“short stack” managed by the SMU 740 may be limited to N entries in acurrent implementation.

It is helpful for managing preemption to have some mechanism thatindicates where the TTU 700 is in traversing the current BVH, in orderto avoid having to start all over again and repeat prior work afterpreemption (which of course would provide no forward progress).

In some example non-limiting approaches, it might be possible to replacea stack with a bit trail that indicates which parts of a binary tree,graph or the like have been traversed. This or other mechanisms could beused by TTU 700 to track where the ray is in traversing the current BVH.Such tracking could result in a data object or other indicator that theTTU 700 can provide to the SM 132 for safekeeping and which the SMeventually returns back to the TTU to enable the TTU to continue whereit left off. Such data will serve two purposes: a representation ofwhere the TTU 700 is in the BVH traversal and how it got there, andallowing changes in that representation to indicate that forwardprogress has been made.

While the preemption mechanism causes the TTU 700 to stop processing allrays in non-limiting implementations, it is possible for the preemptionmechanism to stop the TTU from processing only a subset of rays or evenonly a single ray. For example, in non-limiting example implementations,each TTU 700 could serve two or more SMs 132. If only one of the SMs 132is preempted, it may be desirable to preempt only the rays queried bythe preempted SM while allowing the TTU 700 to continue processing therays from another SM that is not being preempted. In other examples, itwould be possible for the SM 132 to provide more granular control, wherefor example the SM 132 could discontinue its processingthread-by-thread, and command the TTU 700 to stop the work it is doingfor a particular thread. This would provide a more complicated interfaceand would require the SM 132 to keep track of which outstanding rayprocessing request is associated with which thread. It may be moreefficient to, as described above, have the SMU 740 within the TTU 700keep track of which uncompleted rays are associated with whichnot-yet-responded-to queries. In the case of timeout or cycle limits asdescribed above, it may be simpler in some applications for the SM 132to provide targets for each ray to the TTU 700 and allow the TTU totrack all rays with their associated targets.

Example Image Generation Pipeline Including Ray Tracing

The ray tracing and other capabilities described above can be used in avariety of ways. For example, in addition to being used to render ascene using ray tracing, they may be implemented in combination withscan conversion techniques such as in the context of scan convertinggeometric building blocks (i.e., polygon primitives such as triangles)of a 3D model for generating image for display (e.g., on display 150illustrated in FIG. 1). FIG. 14 illustrates an example flowchart forprocessing primitives to provide image pixel values of an image, inaccordance with an embodiment.

As FIG. 14 shows, an image of a 3D model may be generated in response toreceiving a user input (Step 1652). The user input may be a request todisplay an image or image sequence, such as an input operation performedduring interaction with an application (e.g., a game application). Inresponse to the user input, the system performs scan conversion andrasterization of 3D model geometric primitives of a scene usingconventional GPU 3D graphics pipeline (Step 1654). The scan conversionand rasterization of geometric primitives may include for exampleprocessing primitives of the 3D model to determine image pixel valuesusing conventional techniques such as lighting, transforms, texturemapping, rasterization and the like as is well known to those skilled inthe art and discussed below in connection with FIG. 18. The generatedpixel data may be written to a frame buffer.

In step 1656, one or more rays may be traced from one or more points onthe rasterized primitives using TTU hardware acceleration. The rays maybe traced in accordance with the one or more ray-tracing capabilitiesdisclosed in this application. Based on the results of the ray tracing,the pixel values stored in the buffer may be modified (Step 1658).Modifying the pixel values may in some applications for example improvethe image quality by, for example, applying more realistic reflectionsand/or shadows. An image is displayed (Step 1660) using the modifiedpixel values stored in the buffer.

In one example, scan conversion and rasterization of geometricprimitives may be implemented using the processing system described inrelation to FIGS. 15-17, 19, 20, 21 and/or 22, and ray tracing may beimplemented by the SM's 132 using the TTU architecture described inrelation to FIG. 9, to add further visualization features (e.g.,specular reflection, shadows, etc.). FIG. 14 is just a non-limitingexample—the SM's 132 could employ the described TTU by itself withouttexture processing or other conventional 3D graphics processing toproduce images, or the SM's could employ texture processing and otherconventional 3D graphics processing without the described TTU to produceimages. The SM's can also implement any desired image generation orother functionality in software depending on the application to provideany desired programmable functionality that is not bound to the hardwareacceleration features provided by texture mapping hardware, treetraversal hardware or other graphics pipeline hardware.

Example Parallel Processing Architecture Including Ray Tracing

The TTU structure described above can be implemented in, or inassociation with, an example non-limiting parallel processing systemarchitecture such as that described below in relation to FIGS. 15-22.Such a parallel processing architecture can be used for example toimplement the GPU 130 of FIG. 1.

Example Parallel Processing Architecture

FIG. 15 illustrates an example non-limiting parallel processing unit(PPU) 1700. In an embodiment, the PPU 1700 is a multi-threaded processorthat is implemented on one or more integrated circuit devices. The PPU1700 is a latency hiding architecture designed to process many threadsin parallel. A thread (i.e., a thread of execution) is an instantiationof a set of instructions configured to be executed by the PPU 1700. Inan embodiment, the PPU 1700 is configured to implement a graphicsrendering pipeline for processing three-dimensional (3D) graphics datain order to generate two-dimensional (2D) image data for display on adisplay device such as a liquid crystal display (LCD) device, an organiclight emitting diode (OLED) device, a transparent light emitting diode(TOLED) device, a field emission display (FEDs), a field sequentialdisplay, a projection display, a head mounted display or any otherdesired display. In other embodiments, the PPU 1700 may be utilized forperforming general-purpose computations. While one exemplary parallelprocessor is provided herein for illustrative purposes, it should benoted that such processor is set forth for illustrative purposes only,and that any processor may be employed to supplement and/or substitutefor the same.

For example, one or more PPUs 1700 may be configured to acceleratethousands of High Performance Computing (HPC), data center, and machinelearning applications. The PPU 1700 may be configured to acceleratenumerous deep learning systems and applications including autonomousvehicle platforms, deep learning, high-accuracy speech, image, and textrecognition systems, intelligent video analytics, molecular simulations,drug discovery, disease diagnosis, weather forecasting, big dataanalytics, astronomy, molecular dynamics simulation, financial modeling,robotics, factory automation, real-time language translation, onlinesearch optimizations, and personalized user recommendations, and thelike.

The PPU 1700 may be included in a desktop computer, a laptop computer, atablet computer, servers, supercomputers, a smart-phone (e.g., awireless, hand-held device), personal digital assistant (PDA), a digitalcamera, a vehicle, a head mounted display, a hand-held electronicdevice, and the like. In an embodiment, the PPU 1700 is embodied on asingle semiconductor substrate. In another embodiment, the PPU 1700 isincluded in a system-on-a-chip (SoC) along with one or more otherdevices such as additional PPUs 1700, the memory 1704, a reducedinstruction set computer (RISC) CPU, a memory management unit (MMU), adigital-to-analog converter (DAC), and the like.

In an embodiment, the PPU 1700 may be included on a graphics card thatincludes one or more memory devices 1704. The graphics card may beconfigured to interface with a PCIe slot on a motherboard of a desktopcomputer. In yet another embodiment, the PPU 1700 may be an integratedgraphics processing unit (iGPU) or parallel processor included in thechipset of the motherboard.

As shown in FIG. 15, the PPU 1700 includes an Input/Output (I/O) unit1705, a front end unit 1715, a scheduler unit 1720, a work distributionunit 1725, a hub 1730, a crossbar (Xbar) 1770, one or more generalprocessing clusters (GPCs) 1750, and one or more partition units 1780.The PPU 1700 may be connected to a host processor or other PPUs 1700 viaone or more high-speed NVLink 1710 interconnect. The PPU 1700 may beconnected to a host processor or other peripheral devices via aninterconnect 1702. The PPU 1700 may also be connected to a local memorycomprising a number of memory devices 1704. In an embodiment, the localmemory may comprise a number of dynamic random access memory (DRAM)devices. The DRAM devices may be configured as a high-bandwidth memory(HBM) subsystem, with multiple DRAM dies stacked within each device.

The NVLink 1710 interconnect enables systems to scale and include one ormore PPUs 1700 combined with one or more CPUs, supports cache coherencebetween the PPUs 1700 and CPUs, and CPU mastering. Data and/or commandsmay be transmitted by the NVLink 1710 through the hub 1730 to/from otherunits of the PPU 1700 such as one or more copy engines, a video encoder,a video decoder, a power management unit, etc. (not explicitly shown).The NVLink 1710 is described in more detail in conjunction with FIG. 21.

The I/O unit 1705 is configured to transmit and receive communications(i.e., commands, data, etc.) from a host processor (not shown) over theinterconnect 1702. The I/O unit 1705 may communicate with the hostprocessor directly via the interconnect 1702 or through one or moreintermediate devices such as a memory bridge. In an embodiment, the I/Ounit 1705 may communicate with one or more other processors, such as oneor more of the PPUs 1700 via the interconnect 1702. In an embodiment,the I/O unit 1705 implements a Peripheral Component Interconnect Express(PCIe) interface for communications over a PCIe bus and the interconnect1702 is a PCIe bus. In alternative embodiments, the I/O unit 1705 mayimplement other types of well-known interfaces for communicating withexternal devices.

The I/O unit 1705 decodes packets received via the interconnect 1702. Inan embodiment, the packets represent commands configured to cause thePPU 1700 to perform various operations. The I/O unit 1705 transmits thedecoded commands to various other units of the PPU 1700 as the commandsmay specify. For example, some commands may be transmitted to the frontend unit 1715. Other commands may be transmitted to the hub 1730 orother units of the PPU 1700 such as one or more copy engines, a videoencoder, a video decoder, a power management unit, etc. (not explicitlyshown). In other words, the I/O unit 1705 is configured to routecommunications between and among the various logical units of the PPU1700.

In an embodiment, a program executed by the host processor encodes acommand stream in a buffer that provides workloads to the PPU 1700 forprocessing. A workload may comprise several instructions and data to beprocessed by those instructions. The buffer is a region in a memory thatis accessible (i.e., read/write) by both the host processor and the PPU1700. For example, the I/O unit 1705 may be configured to access thebuffer in a system memory connected to the interconnect 1702 via memoryrequests transmitted over the interconnect 1702. In an embodiment, thehost processor writes the command stream to the buffer and thentransmits a pointer to the start of the command stream to the PPU 1700.The front end unit 1715 receives pointers to one or more commandstreams. The front end unit 1715 manages the one or more streams,reading commands from the streams and forwarding commands to the variousunits of the PPU 1700.

The front end unit 1715 is coupled to a scheduler unit 1720 thatconfigures the various GPCs 1750 to process tasks defined by the one ormore streams. The scheduler unit 1720 is configured to track stateinformation related to the various tasks managed by the scheduler unit1720. The state may indicate which GPC 1750 a task is assigned to,whether the task is active or inactive, a priority level associated withthe task, and so forth. The scheduler unit 1720 manages the execution ofa plurality of tasks on the one or more GPCs 1750.

The scheduler unit 1720 is coupled to a work distribution unit 1725 thatis configured to dispatch tasks for execution on the GPCs 1750. The workdistribution unit 1725 may track a number of scheduled tasks receivedfrom the scheduler unit 1720. In an embodiment, the work distributionunit 1725 manages a pending task pool and an active task pool for eachof the GPCs 1750. The pending task pool may comprise a number of slots(e.g., 32 slots) that contain tasks assigned to be processed by aparticular GPC 1750. The active task pool may comprise a number of slots(e.g., 4 slots) for tasks that are actively being processed by the GPCs1750. As a GPC 1750 finishes the execution of a task, that task isevicted from the active task pool for the GPC 1750 and one of the othertasks from the pending task pool is selected and scheduled for executionon the GPC 1750. If an active task has been idle on the GPC 1750, suchas while waiting for a data dependency to be resolved, then the activetask may be evicted from the GPC 1750 and returned to the pending taskpool while another task in the pending task pool is selected andscheduled for execution on the GPC 1750.

The work distribution unit 1725 communicates with the one or more GPCs1750 via XBar 1770. The XBar 1770 is an interconnect network thatcouples many of the units of the PPU 1700 to other units of the PPU1700. For example, the XBar 1770 may be configured to couple the workdistribution unit 1725 to a particular GPC 1750. Although not shownexplicitly, one or more other units of the PPU 1700 may also beconnected to the XBar 1770 via the hub 1730.

The tasks are managed by the scheduler unit 1720 and dispatched to a GPC1750 by the work distribution unit 1725. The GPC 1750 is configured toprocess the task and generate results. The results may be consumed byother tasks within the GPC 1750, routed to a different GPC 1750 via theXBar 1770, or stored in the memory 1704. The results can be written tothe memory 1704 via the partition units 1780, which implement a memoryinterface for reading and writing data to/from the memory 1704. Theresults can be transmitted to another PPU 1704 or CPU via the NVLink1710. In an embodiment, the PPU 1700 includes a number U of partitionunits 1780 that is equal to the number of separate and distinct memorydevices 1704 coupled to the PPU 1700. A partition unit 1780 will bedescribed in more detail below in conjunction with FIG. 16.

In an embodiment, a host processor (e.g., processor 120 of FIG. 1)executes a driver kernel that implements an application programminginterface (API) that enables one or more applications executing on thehost processor to schedule operations for execution on the PPU 1700. Inan embodiment, multiple compute applications are simultaneously executedby the PPU 1700 and the PPU 1700 provides isolation, quality of service(QoS), and independent address spaces for the multiple computeapplications. An application may generate instructions (i.e., API calls)that cause the driver kernel to generate one or more tasks for executionby the PPU 1700. The driver kernel outputs tasks to one or more streamsbeing processed by the PPU 1700. Each task may comprise one or moregroups of related threads, referred to herein as a warp. In anembodiment, a warp comprises 32 related threads that may be executed inparallel. Cooperating threads may refer to a plurality of threadsincluding instructions to perform the task and that may exchange datathrough shared memory. Threads and cooperating threads are described inmore detail in conjunction with FIG. 19.

Example Memory Partition Unit

The MMU 1890 provides an interface between the GPC 1750 and thepartition unit 1780. The MMU 1890 may provide translation of virtualaddresses into physical addresses, memory protection, and arbitration ofmemory requests. In an embodiment, the MMU 1890 provides one or moretranslation lookaside buffers (TLBs) for performing translation ofvirtual addresses into physical addresses in the memory 1704.

FIG. 16 illustrates a memory partition unit 1780 of the PPU 1700 of FIG.15, in accordance with an embodiment. As shown in FIG. 16, the memorypartition unit 1780 includes a Raster Operations (ROP) unit 1850, alevel two (L2) cache 1860, and a memory interface 1870. The memoryinterface 1870 is coupled to the memory 1704. Memory interface 1870 mayimplement 32, 64, 128, 1024-bit data buses, or the like, for high-speeddata transfer. In an embodiment, the PPU 1700 incorporates U memoryinterfaces 1870, one memory interface 1870 per pair of partition units1780, where each pair of partition units 1780 is connected to acorresponding memory device 1704. For example, PPU 1700 may be connectedto up to Y memory devices 1704, such as high bandwidth memory stacks orgraphics double-data-rate, version 5, synchronous dynamic random accessmemory, or other types of persistent storage.

In an embodiment, the memory interface 1870 implements an HBM2 memoryinterface and Y equals half U. In an embodiment, the HBM2 memory stacksare located on the same physical package as the PPU 1700, providingsubstantial power and area savings compared with conventional GDDR5SDRAM systems. In an embodiment, each HBM2 stack includes four memorydies and Y equals 4, with HBM2 stack including two 128-bit channels perdie for a total of 8 channels and a data bus width of 1024 bits.

In an embodiment, the memory 1704 supports Single-Error CorrectingDouble-Error Detecting (SECDED) Error Correction Code (ECC) to protectdata. ECC provides higher reliability for compute applications that aresensitive to data corruption. Reliability is especially important inlarge-scale cluster computing environments where PPUs 1700 process verylarge datasets and/or run applications for extended periods.

In an embodiment, the PPU 1700 implements a multi-level memoryhierarchy. In an embodiment, the memory partition unit 1780 supports aunified memory to provide a single unified virtual address space for CPUand PPU 1700 memory, enabling data sharing between virtual memorysystems. In an embodiment the frequency of accesses by a PPU 1700 tomemory located on other processors is traced to ensure that memory pagesare moved to the physical memory of the PPU 1700 that is accessing thepages more frequently. In an embodiment, the NVLink 1710 supportsaddress translation services allowing the PPU 1700 to directly access aCPU's page tables and providing full access to CPU memory by the PPU1700.

In an embodiment, copy engines transfer data between multiple PPUs 1700or between PPUs 1700 and CPUs. The copy engines can generate page faultsfor addresses that are not mapped into the page tables. The memorypartition unit 1780 can then service the page faults, mapping theaddresses into the page table, after which the copy engine can performthe transfer. In a conventional system, memory is pinned (i.e.,non-pageable) for multiple copy engine operations between multipleprocessors, substantially reducing the available memory. With hardwarepage faulting, addresses can be passed to the copy engines withoutworrying if the memory pages are resident, and the copy process istransparent.

Data from the memory 1704 or other system memory may be fetched by thememory partition unit 1780 and stored in the L2 cache 1860, which islocated on-chip and is shared between the various GPCs 1750. As shown,each memory partition unit 1780 includes a portion of the L2 cache 1860associated with a corresponding memory device 1704. Lower level cachesmay then be implemented in various units within the GPCs 1750. Forexample, each of the SMs 1840 may implement a level one (L1) cache. TheL1 cache is private memory that is dedicated to a particular SM 1840.Data from the L2 cache 1860 may be fetched and stored in each of the L1caches for processing in the functional units of the SMs 1840. The L2cache 1860 is coupled to the memory interface 1870 and the XBar 1770.

The ROP unit 1850 performs graphics raster operations related to pixelcolor, such as color compression, pixel blending, and the like. The ROPunit 1850 also implements depth testing in conjunction with the rasterengine 1825, receiving a depth for a sample location associated with apixel fragment from the culling engine of the raster engine 1825. Thedepth is tested against a corresponding depth in a depth buffer for asample location associated with the fragment. If the fragment passes thedepth test for the sample location, then the ROP unit 1850 updates thedepth buffer and transmits a result of the depth test to the rasterengine 1825. It will be appreciated that the number of partition units1780 may be different than the number of GPCs 1750 and, therefore, eachROP unit 1850 may be coupled to each of the GPCs 1750. The ROP unit 1850tracks packets received from the different GPCs 1750 and determineswhich GPC 1750 that a result generated by the ROP unit 1850 is routed tothrough the Xbar 1770. Although the ROP unit 1850 is included within thememory partition unit 1780 in FIG. 16, in other embodiment, the ROP unit1850 may be outside of the memory partition unit 1780. For example, theROP unit 1850 may reside in the GPC 1750 or another unit.

Example General Processing Clusters

FIG. 17 illustrates a GPC 1750 of the PPU 1700 of FIG. 15, in accordancewith an embodiment. As shown in FIG. 17, each GPC 1750 includes a numberof hardware units for processing tasks. In an embodiment, each GPC 1750includes a pipeline manager 1810, a pre-raster operations unit (PROP)1815, a raster engine 1825, a work distribution crossbar (WDX) 1880, amemory management unit (MMU) 1890, and one or more Data ProcessingClusters (DPCs) 1820. It will be appreciated that the GPC 1750 of FIG.17 may include other hardware units in lieu of or in addition to theunits shown in FIG. 17.

In an embodiment, the operation of the GPC 1750 is controlled by thepipeline manager 1810. The pipeline manager 1810 manages theconfiguration of the one or more DPCs 1820 for processing tasksallocated to the GPC 1750. In an embodiment, the pipeline manager 1810may configure at least one of the one or more DPCs 1820 to implement atleast a portion of a graphics rendering pipeline.

Each DPC 1820 included in the GPC 1750 includes an M-Pipe Controller(MPC) 1830, a primitive engine 1835, one or more SMs 1840, one or moreTexture Units 1842, and one or more TTUs 700. The SM 1840 may bestructured similarly to SM 132 described above. The MPC 1830 controlsthe operation of the DPC 1820, routing packets received from thepipeline manager 1810 to the appropriate units in the DPC 1820. Forexample, packets associated with a vertex may be routed to the primitiveengine 1835, which is configured to fetch vertex attributes associatedwith the vertex from the memory 1704. In contrast, packets associatedwith a shader program may be transmitted to the SM 1840.

When configured for general purpose parallel computation, a simplerconfiguration can be used compared with graphics processing.Specifically, the fixed function graphics processing units shown in FIG.15, are bypassed, creating a much simpler programming model. In thegeneral purpose parallel computation configuration, the workdistribution unit 1725 assigns and distributes blocks of threadsdirectly to the DPCs 1820. The threads in a block execute the sameprogram, using a unique thread ID in the calculation to ensure eachthread generates unique results, using the SM 1840 to execute theprogram and perform calculations, shared memory/L1 cache 1970 tocommunicate between threads, and the LSU 1954 to read and write globalmemory through the shared memory/L1 cache 1970 and the memory partitionunit 1780. When configured for general purpose parallel computation, theSM 1840 can also write commands that the scheduler unit 1720 can use tolaunch new work on the DPCs 1820. The TTU 700 can be used to acceleratespatial queries in the context of general purpose computation.

Graphics Rendering Pipeline

A DPC 1820 may be configured to execute a vertex shader program on theprogrammable streaming multiprocessor (SM) 1840 which may acceleratecertain shading operations with TTU 700. The pipeline manager 1810 mayalso be configured to route packets received from the work distributionunit 1725 to the appropriate logical units within the GPC 1750. Forexample, some packets may be routed to fixed function hardware units inthe PROP 1815 and/or raster engine 1825 while other packets may berouted to the DPCs 1820 for processing by the primitive engine 1835 orthe SM 1840. In an embodiment, the pipeline manager 1810 may configureat least one of the one or more DPCs 1820 to implement a neural networkmodel and/or a computing pipeline.

The PROP unit 1815 is configured to route data generated by the rasterengine 1825 and the DPCs 1820 to a Raster Operations (ROP) unit,described in more detail in conjunction with FIG. 16. The PROP unit 1815may also be configured to perform optimizations for color blending,organize pixel data, perform address translations, and the like.

The raster engine 1825 includes a number of fixed function hardwareunits configured to perform various raster operations. In an embodiment,the raster engine 1825 includes a setup engine, a coarse raster engine,a culling engine, a clipping engine, a fine raster engine, and a tilecoalescing engine. The setup engine receives transformed vertices andgenerates plane equations associated with the geometric primitivedefined by the vertices. The plane equations are transmitted to thecoarse raster engine to generate coverage information (e.g., an x,ycoverage mask for a tile) for the primitive. The output of the coarseraster engine is transmitted to the culling engine where fragmentsassociated with the primitive that fail a z-test are culled, andnon-culled fragments are transmitted to a clipping engine wherefragments lying outside a viewing frustum are clipped. Those fragmentsthat survive clipping and culling may be passed to the fine rasterengine to generate attributes for the pixel fragments based on the planeequations generated by the setup engine. The output of the raster engine1825 comprises fragments to be processed, for example, by a fragmentshader implemented within a DPC 1820

In more detail, the PPU 1700 is configured to receive commands thatspecify shader programs for processing graphics data. Graphics data maybe defined as a set of primitives such as points, lines, triangles,quads, triangle strips, and the like. Typically, a primitive includesdata that specifies a number of vertices for the primitive (e.g., in amodel-space coordinate system) as well as attributes associated witheach vertex of the primitive. The PPU 1700 can be configured to processthe graphics primitives to generate a frame buffer (i.e., pixel data foreach of the pixels of the display) using for example TTU 700 as ahardware acceleration resource.

An application writes model data for a scene (i.e., a collection ofvertices and attributes) to a memory such as a system memory or memory1704. The model data defines each of the objects that may be visible ona display. The model data may also define one or more BVH's as describedabove. The application then makes an API call to the driver kernel thatrequests the model data to be rendered and displayed. The driver kernelreads the model data and writes commands to the one or more streams toperform operations to process the model data. The commands may referencedifferent shader programs to be implemented on the SMs 1840 of the PPU1700 including one or more of a vertex shader, hull shader, domainshader, geometry shader, a pixel shader, a ray generation shader, a rayintersection shader, a ray hit shader, and a ray miss shader (thesecorrespond to the shaders defined by the DXR API, ignoring anydistinction between “closest-hit” and “any-hit” shaders; seehttps://devblogs.nvidia.com/introduction-nvidia-rtx-directx-ray-tracing/).For example, one or more of the SMs 1840 may be configured to execute avertex shader program that processes a number of vertices defined by themodel data. In an embodiment, the different SMs 1840 may be configuredto execute different shader programs concurrently. For example, a firstsubset of SMs 1840 may be configured to execute a vertex shader programwhile a second subset of SMs 1840 may be configured to execute a pixelshader program. The first subset of SMs 1840 processes vertex data toproduce processed vertex data and writes the processed vertex data tothe L2 cache 1860 and/or the memory 1704 (see FIG. 16). After theprocessed vertex data is rasterized (i.e., transformed fromthree-dimensional data into two-dimensional data in screen space) toproduce fragment data, the second subset of SMs 1840 executes a pixelshader to produce processed fragment data, which is then blended withother processed fragment data and written to the frame buffer in memory1704. The vertex shader program and pixel shader program may executeconcurrently, processing different data from the same scene in apipelined fashion until all of the model data for the scene has beenrendered to the frame buffer. Then, the contents of the frame buffer aretransmitted to a display controller for display on a display device.

FIG. 18 is a conceptual diagram of a graphics processing pipeline 2000implemented by the PPU 1700 of FIG. 15. The graphics processing pipeline2000 is an abstract flow diagram of the processing steps implemented togenerate 2D computer-generated images from 3D geometry data. As iswell-known, pipeline architectures may perform long latency operationsmore efficiently by splitting up the operation into a plurality ofstages, where the output of each stage is coupled to the input of thenext successive stage. Thus, the graphics processing pipeline 2000receives input data 2001 that is transmitted from one stage to the nextstage of the graphics processing pipeline 2000 to generate output data2002. In an embodiment, the graphics processing pipeline 2000 mayrepresent a graphics processing pipeline defined by the OpenGL® API. Asan option, the graphics processing pipeline 2000 may be implemented inthe context of the functionality and architecture of the previousFigures and/or any subsequent Figure(s). As discussed above withreference to FIG. 14, the ray tracing may be used to improve the imagequality generated by rasterization of geometric primitives. In someembodiments, ray tracing operations and TTU structure disclosed in thisapplication may be applied to one or more states of the graphicsprocessing pipeline 2000 to improve the subjective image quality.

As shown in FIG. 18, the graphics processing pipeline 2000 comprises apipeline architecture that includes a number of stages. The stagesinclude, but are not limited to, a data assembly stage 2010, a vertexshading stage 2020, a primitive assembly stage 2030, a geometry shadingstage 2040, a viewport scale, cull, and clip (VSCC) stage 2050, arasterization stage 2060, a fragment shading stage 2070, and a rasteroperations stage 2080. In an embodiment, the input data 2001 comprisescommands that configure the processing units to implement the stages ofthe graphics processing pipeline 2000 and geometric primitives (e.g.,points, lines, triangles, quads, triangle strips or fans, etc.) to beprocessed by the stages. The output data 2002 may comprise pixel data(i.e., color data) that is copied into a frame buffer or other type ofsurface data structure in a memory.

The data assembly stage 2010 receives the input data 2001 that specifiesvertex data for high-order surfaces, primitives, or the like. The dataassembly stage 2010 collects the vertex data in a temporary storage orqueue, such as by receiving a command from the host processor thatincludes a pointer to a buffer in memory and reading the vertex datafrom the buffer. The vertex data is then transmitted to the vertexshading stage 2020 for processing.

The vertex shading stage 2020 processes vertex data by performing a setof operations (i.e., a vertex shader or a program) once for each of thevertices. Vertices may be, e.g., specified as a 4-coordinate vector(i.e., <x, y, z, w>) associated with one or more vertex attributes(e.g., color, texture coordinates, surface normal, etc.). The vertexshading stage 2020 may manipulate individual vertex attributes such asposition, color, texture coordinates, and the like. In other words, thevertex shading stage 2020 performs operations on the vertex coordinatesor other vertex attributes associated with a vertex. Such operationscommonly including lighting operations (i.e., modifying color attributesfor a vertex) and transformation operations (i.e., modifying thecoordinate space for a vertex). For example, vertices may be specifiedusing coordinates in an object-coordinate space, which are transformedby multiplying the coordinates by a matrix that translates thecoordinates from the object-coordinate space into a world space or anormalized-device-coordinate (NCD) space. The vertex shading stage 2020generates transformed vertex data that is transmitted to the primitiveassembly stage 2030.

The primitive assembly stage 2030 collects vertices output by the vertexshading stage 2020 and groups the vertices into geometric primitives forprocessing by the geometry shading stage 2040. For example, theprimitive assembly stage 2030 may be configured to group every threeconsecutive vertices as a geometric primitive (i.e., a triangle) fortransmission to the geometry shading stage 2040. In some embodiments,specific vertices may be reused for consecutive geometric primitives(e.g., two consecutive triangles in a triangle strip may share twovertices). The primitive assembly stage 2030 transmits geometricprimitives (i.e., a collection of associated vertices) to the geometryshading stage 2040.

The geometry shading stage 2040 processes geometric primitives byperforming a set of operations (i.e., a geometry shader or program) onthe geometric primitives. Tessellation operations may generate one ormore geometric primitives from each geometric primitive. In other words,the geometry shading stage 2040 may subdivide each geometric primitiveinto a finer mesh of two or more geometric primitives for processing bythe rest of the graphics processing pipeline 2000. The geometry shadingstage 2040 transmits geometric primitives to the viewport SCC stage2050.

In an embodiment, the graphics processing pipeline 2000 may operatewithin a streaming multiprocessor and the vertex shading stage 2020, theprimitive assembly stage 2030, the geometry shading stage 2040, thefragment shading stage 2070, a ray tracing shader, and/orhardware/software associated therewith, may sequentially performprocessing operations. Once the sequential processing operations arecomplete, in an embodiment, the viewport SCC stage 2050 may utilize thedata. In an embodiment, primitive data processed by one or more of thestages in the graphics processing pipeline 2000 may be written to acache (e.g. L1 cache, a vertex cache, etc.). In this case, in anembodiment, the viewport SCC stage 2050 may access the data in thecache. In an embodiment, the viewport SCC stage 2050 and therasterization stage 2060 are implemented as fixed function circuitry.

The viewport SCC stage 2050 performs viewport scaling, culling, andclipping of the geometric primitives. Each surface being rendered to isassociated with an abstract camera position. The camera positionrepresents a location of a viewer looking at the scene and defines aviewing frustum that encloses the objects of the scene. The viewingfrustum may include a viewing plane, a rear plane, and four clippingplanes. Any geometric primitive entirely outside of the viewing frustummay be culled (i.e., discarded) because the geometric primitive will notcontribute to the final rendered scene. Any geometric primitive that ispartially inside the viewing frustum and partially outside the viewingfrustum may be clipped (i.e., transformed into a new geometric primitivethat is enclosed within the viewing frustum. Furthermore, geometricprimitives may each be scaled based on a depth of the viewing frustum.All potentially visible geometric primitives are then transmitted to therasterization stage 2060.

The rasterization stage 2060 converts the 3D geometric primitives into2D fragments (e.g. capable of being utilized for display, etc.). Therasterization stage 2060 may be configured to utilize the vertices ofthe geometric primitives to setup a set of plane equations from whichvarious attributes can be interpolated. The rasterization stage 2060 mayalso compute a coverage mask for a plurality of pixels that indicateswhether one or more sample locations for the pixel intercept thegeometric primitive. In an embodiment, z-testing may also be performedto determine if the geometric primitive is occluded by other geometricprimitives that have already been rasterized. The rasterization stage2060 generates fragment data (i.e., interpolated vertex attributesassociated with a particular sample location for each covered pixel)that are transmitted to the fragment shading stage 2070.

The fragment shading stage 2070 processes fragment data by performing aset of operations (i.e., a fragment shader or a program) on each of thefragments. The fragment shading stage 2070 may generate pixel data(i.e., color values) for the fragment such as by performing lightingoperations or sampling texture maps using interpolated texturecoordinates for the fragment. The fragment shading stage 2070 generatespixel data that is transmitted to the raster operations stage 2080.

The raster operations stage 2080 may perform various operations on thepixel data such as performing alpha tests, stencil tests, and blendingthe pixel data with other pixel data corresponding to other fragmentsassociated with the pixel. When the raster operations stage 2080 hasfinished processing the pixel data (i.e., the output data 2002), thepixel data may be written to a render target such as a frame buffer, acolor buffer, or the like.

It will be appreciated that one or more additional stages may beincluded in the graphics processing pipeline 2000 in addition to or inlieu of one or more of the stages described above. Variousimplementations of the abstract graphics processing pipeline mayimplement different stages. Furthermore, one or more of the stagesdescribed above may be excluded from the graphics processing pipeline insome embodiments (such as the geometry shading stage 2040). Other typesof graphics processing pipelines are contemplated as being within thescope of the present disclosure. Furthermore, any of the stages of thegraphics processing pipeline 2000 may be implemented by one or morededicated hardware units within a graphics processor such as PPU 200.Other stages of the graphics processing pipeline 2000 may be implementedby programmable hardware units such as the SM 1840 of the PPU 1700.

The graphics processing pipeline 2000 may be implemented via anapplication executed by a host processor, such as a CPU 120. In anembodiment, a device driver may implement an application programminginterface (API) that defines various functions that can be utilized byan application in order to generate graphical data for display. Thedevice driver is a software program that includes a plurality ofinstructions that control the operation of the PPU 1700. The APIprovides an abstraction for a programmer that lets a programmer utilizespecialized graphics hardware, such as the PPU 1700, to generate thegraphical data without requiring the programmer to utilize the specificinstruction set for the PPU 1700. The application may include an APIcall that is routed to the device driver for the PPU 1700. The devicedriver interprets the API call and performs various operations torespond to the API call. In some instances, the device driver mayperform operations by executing instructions on the CPU. In otherinstances, the device driver may perform operations, at least in part,by launching operations on the PPU 1700 utilizing an input/outputinterface between the CPU and the PPU 1700. In an embodiment, the devicedriver is configured to implement the graphics processing pipeline 2000utilizing the hardware of the PPU 1700.

Various programs may be executed within the PPU 1700 in order toimplement the various stages of the graphics processing pipeline 2000.For example, the device driver may launch a kernel on the PPU 1700 toperform the vertex shading stage 2020 on one SM 1840 (or multiple SMs1840). The device driver (or the initial kernel executed by the PPU1800) may also launch other kernels on the PPU 1800 to perform otherstages of the graphics processing pipeline 2000, such as the geometryshading stage 2040 and the fragment shading stage 2070. In addition,some of the stages of the graphics processing pipeline 2000 may beimplemented on fixed unit hardware such as a rasterizer or a dataassembler implemented within the PPU 1800. It will be appreciated thatresults from one kernel may be processed by one or more interveningfixed function hardware units before being processed by a subsequentkernel on an SM 1840.

Example Streaming Multiprocessor

The SM 1840 comprises a programmable streaming processor that isconfigured to process tasks represented by a number of threads. Each SM1840 is multi-threaded and configured to execute a plurality of threads(e.g., 32 threads comprising a warp) from a particular group of threadsconcurrently. In an embodiment, the SM 1840 implements a SIMD(Single-Instruction, Multiple-Data) architecture where each thread in agroup of threads (i.e., a warp) is configured to process a different setof data based on the same set of instructions. All threads in the groupof threads execute the same instructions. In another embodiment, the SM1840 implements a SIMT (Single-Instruction, Multiple Thread)architecture where each thread in a group of threads is configured toprocess a different set of data based on the same set of instructions,but where individual threads in the group of threads are allowed todiverge during execution. In an embodiment, a program counter, callstack, and execution state is maintained for each warp, enablingconcurrency between warps and serial execution within warps when threadswithin the warp diverge. In another embodiment, a program counter, callstack, and execution state is maintained for each individual thread,enabling equal concurrency between all threads, within and betweenwarps. When execution state is maintained for each individual thread,threads executing the same instructions may be converged and executed inparallel for maximum efficiency.

FIG. 19 illustrates the streaming multi-processor 1840 of FIG. 17, inaccordance with an embodiment. As shown in FIG. 19, the SM 1840 includesan instruction cache 1905, one or more scheduler units 1910, a registerfile 1920, one or more processing cores 1950, one or more specialfunction units (SFUs) 1952, one or more load/store units (LSUs) 1954, aninterconnect network 1980, a shared memory/L1 cache 1970.

As described above, the work distribution unit 1725 dispatches tasks forexecution on the GPCs 1750 of the PPU 1700. The tasks are allocated to aparticular DPC 1820 within a GPC 1750 and, if the task is associatedwith a shader program, the task may be allocated to an SM 1840. Thescheduler unit 1910 receives the tasks from the work distribution unit1725 and manages instruction scheduling for one or more thread blocksassigned to the SM 1840. The scheduler unit 1910 schedules thread blocksfor execution as warps of parallel threads, where each thread block isallocated at least one warp. In an embodiment, each warp executes 32threads. The scheduler unit 1910 may manage a plurality of differentthread blocks, allocating the warps to the different thread blocks andthen dispatching instructions from the plurality of differentcooperative groups to the various functional units (i.e., cores 1950,SFUs 1952, and LSUs 1954) during each clock cycle.

Cooperative Groups is a programming model for organizing groups ofcommunicating threads that allows developers to express the granularityat which threads are communicating, enabling the expression of richer,more efficient parallel decompositions. Cooperative launch APIs supportsynchronization amongst thread blocks for the execution of parallelalgorithms. Conventional programming models provide a single, simpleconstruct for synchronizing cooperating threads: a barrier across allthreads of a thread block (i.e., the syncthreads( ) function). However,programmers would often like to define groups of threads at smaller thanthread block granularities and synchronize within the defined groups toenable greater performance, design flexibility, and software reuse inthe form of collective group-wide function interfaces.

Cooperative Groups enables programmers to define groups of threadsexplicitly at sub-block (i.e., as small as a single thread) andmulti-block granularities, and to perform collective operations such assynchronization on the threads in a cooperative group. The programmingmodel supports clean composition across software boundaries, so thatlibraries and utility functions can synchronize safely within theirlocal context without having to make assumptions about convergence.Cooperative Groups primitives enable new patterns of cooperativeparallelism, including producer-consumer parallelism, opportunisticparallelism, and global synchronization across an entire grid of threadblocks.

A dispatch unit 1915 is configured to transmit instructions to one ormore of the functional units. In the embodiment, the scheduler unit 1910includes two dispatch units 1915 that enable two different instructionsfrom the same warp to be dispatched during each clock cycle. Inalternative embodiments, each scheduler unit 1910 may include a singledispatch unit 1915 or additional dispatch units 1915.

Each SM 1840 includes a register file 1920 that provides a set ofregisters for the functional units of the SM 1840. In an embodiment, theregister file 1920 is divided between each of the functional units suchthat each functional unit is allocated a dedicated portion of theregister file 1920. In another embodiment, the register file 1920 isdivided between the different warps being executed by the SM 1840. Theregister file 1920 provides temporary storage for operands connected tothe data paths of the functional units. FIG. 20 illustrates an exampleconfiguration of the registers files in the SM 1840.

Each SM 1840 comprises L processing cores 1950. In an embodiment, the SM1840 includes a large number (e.g., 128, etc.) of distinct processingcores 1950. Each core 1950 may include a fully-pipelined,single-precision, double-precision, and/or mixed precision processingunit that includes a floating point arithmetic logic unit and an integerarithmetic logic unit. In an embodiment, the floating point arithmeticlogic units implement the IEEE 754-2008 standard for floating pointarithmetic. In an embodiment, the cores 1950 include 64 single-precision(32-bit) floating point cores, 64 integer cores, 32 double-precision(64-bit) floating point cores, and 8 tensor cores.

Tensor cores are configured to perform matrix operations, and, in anembodiment, one or more tensor cores are included in the cores 1950. Inparticular, the tensor cores are configured to perform deep learningmatrix arithmetic, such as convolution operations for neural networktraining and inferencing. In an embodiment, each tensor core operates ona 4×4 matrix and performs a matrix multiply and accumulate operationD=A×B+C, where A, B, C, and D are 4×4 matrices.

In an embodiment, the matrix multiply inputs A and B are 16-bit floatingpoint matrices, while the accumulation matrices C and D may be 16-bitfloating point or 32-bit floating point matrices. Tensor Cores operateon 16-bit floating point input data with 32-bit floating pointaccumulation. The 16-bit floating point multiply requires 64 operationsand results in a full precision product that is then accumulated using32-bit floating point addition with the other intermediate products fora 4×4×4 matrix multiply. In practice, Tensor Cores are used to performmuch larger two-dimensional or higher dimensional matrix operations,built up from these smaller elements. An API, such as CUDA 9 C++ API,exposes specialized matrix load, matrix multiply and accumulate, andmatrix store operations to efficiently use Tensor Cores from a CUDA-C++program. At the CUDA level, the warp-level interface assumes 16×16 sizematrices spanning all 32 threads of the warp.

Each SM 1840 also comprises M SFUs 1952 that perform special functions(e.g., attribute evaluation, reciprocal square root, and the like). Inan embodiment, the SFUs 1952 may include a tree traversal unitconfigured to traverse a hierarchical tree data structure. In anembodiment, the SFUs 1952 may include texture unit configured to performtexture map filtering operations. In an embodiment, the texture unitsare configured to load texture maps (e.g., a 2D array of texels) fromthe memory 1704 and sample the texture maps to produce sampled texturevalues for use in shader programs executed by the SM 1840. In anembodiment, the texture maps are stored in the shared memory/L1 cache1970. The texture units implement texture operations such as filteringoperations using mip-maps (i.e., texture maps of varying levels ofdetail). In an embodiment, each SM 1740 includes two texture units.

Each SM 1840 also comprises N LSUs 1954 that implement load and storeoperations between the shared memory/L1 cache 1970 and the register file1920. Each SM 1840 includes an interconnect network 1980 that connectseach of the functional units to the register file 1920 and the LSU 1954to the register file 1920, shared memory/L1 cache 1970. In anembodiment, the interconnect network 1980 is a crossbar that can beconfigured to connect any of the functional units to any of theregisters in the register file 1920 and connect the LSUs 1954 to theregister file and memory locations in shared memory/L1 cache 1970.

The shared memory/L1 cache 1970 is an array of on-chip memory thatallows for data storage and communication between the SM 1840 and theprimitive engine 1835 and between threads in the SM 1840. In anembodiment, the shared memory/L1 cache 1970 comprises 128 KB of storagecapacity and is in the path from the SM 1840 to the partition unit 1780.The shared memory/L1 cache 1970 can be used to cache reads and writes.One or more of the shared memory/L1 cache 1970, L2 cache 1860, andmemory 1704 are backing stores.

Combining data cache and shared memory functionality into a singlememory block provides the best overall performance for both types ofmemory accesses. The capacity is usable as a cache by programs that donot use shared memory. For example, if shared memory is configured touse half of the capacity, texture and load/store operations can use theremaining capacity. Integration within the shared memory/L1 cache 1970enables the shared memory/L1 cache 1970 to function as a high-throughputconduit for streaming data while simultaneously providing high-bandwidthand low-latency access to frequently reused data.

FIG. 20 illustrates one example architecture for the SM 1840. Asillustrated in FIG. 17, the SM 1840 may be coupled to one or moreTexture Unit 1842 and/or one or more TTUs 700. As a compromise betweenperformance and area, one example non-limiting embodiment may include asingle Texture Unit 1842 and/or a single TTU 700 per groups of SMs 1840(e.g., See FIG. 17). The TTU 700 may communicate with the SMs 1840 via aTTU input/output block in memory input-output and with a L1 cache via adedicated read interface. In one example embodiment, the TTU 700 onlyreads from the main memory and does not write to the main memory.

Example More Detailed TTU Architecture

As discussed above, the TTU 700 may be a coprocessor to the SM 1840.Like a texture processor, it is exposed via a set of SM instructions,accesses memory as a read-only client of the L1 cache, and returnsresults into the SM register file. Unlike some texture processors, theamount of data that may need to be passed into and out of the TTU 700for a typical query makes it difficult in some embodiments to specifyall the source and destination registers in a single instruction (andbecause most of this data is unique per-thread, there is no TTU analogueof texture headers and samplers). As a consequence, the TTU 700 in someembodiments is programmed via a multi-instruction sequence. Thissequence can be conceptualized as a single “macro-instruction” in someimplementations.

Also like a Texture Units 1842, the TTU 700 in some implementations mayrely on certain read-only data structures in memory that areprepopulated by software. These include:

-   -   One or more BVHs, where each BVH is for example a tree of        axis-aligned bounding boxes, stored in a compressed format that        greatly reduces memory traffic compared to an uncompressed        representation. Each node in the BVH is stored as a complet        structure, with size and alignment in some implementations        matched to that of an L1 cache line. Child complets of a given        parent are preferably stored contiguously in memory and child        pointers are stored in compressed form.    -   Zero or more instance nodes, which provide a way to connect a        leaf of one BVH to the root of another. An instance node may be        a data structure that is also aligned. This structure may        contain a pointer to the sub-BVH, flags that affect back-face        culling behavior in the sub-BVH, and a matrix that corresponds        to the first three rows of an arbitrary transformation matrix        (in homogeneous coordinates) from the coordinate system of the        top-level BVH (commonly “world space”) to that of the sub-BVH        (commonly “object space”). The final row of the matrix in some        embodiments is in some implementations implicitly (0, 0, 0, 1).    -   Zero or more triangle or other primitive buffers, containing for        example triangles stored either as a triplet of coordinates per        vertex or in a lossless compressed format understood by the TTU        700. In addition, an alpha bit may be provided per triangle or        other primitive, indicating triangles that require special        handling by software to determine whether the triangle is        actually intersected by a given ray. Triangle buffers can be        organized into blocks. There may also be a per-triangle        force-no-cull function bit. When set, that bit indicates that        both sides of the triangle should be treated as front-facing or        back-facing with respect to culling, i.e., the triangle should        not be culled because the ray intersects the “back” instead of        the “front”. The simplest use case for this is a single triangle        used to represent a leaf, where we can still see the leaf if the        ray hits it on the back surface.

The TTU 700 in some embodiments is stateless, meaning that noarchitectural state is maintained in the TTU between queries. At thesame time, it is often useful for software running on the SM 1840 torequest continuation of a previous query, which implies that relevantstate should be written to registers by the TTU 700 and then passed backto the TTU in registers (often in-place) to continue. This state maytake the form of a traversal stack that tracks progress in the traversalof the BVH.

A small number of stack initializers may also be provided for beginninga new query of a given type, for example:

-   -   Traversal starting from a complet    -   Intersection of a ray with a range of triangles    -   Intersection of a ray with a range of triangles, followed by        traversal starting from a complet    -   Vertex fetch from a triangle buffer for a given triangle    -   Optional support for instance transforms in front of the        “traversal starting from a complet” and “intersection of a ray        with a range of triangles”.

Vertex fetch is a simple query that may be specified, with request datathat consists of a stack initializer and nothing else. Other query typesmay require the specification of a ray or beam, along with the stack orstack initializer and various ray flags describing details of the query.A ray is given by its three-coordinate origin, three-coordinatedirection, and minimum and maximum values for the t-parameter along theray. A beam is additionally given by a second origin and direction.

Various ray flags can be used to control various aspects of traversalbehavior, back-face culling, and handling of the various child nodetypes, subject to a pass/fail status of an optional rayOp test. RayOpsadd considerable flexibility to the capabilities of the TTU. In someexample embodiments, the RayOps portion introduces two Ray Flag versionscan be dynamically selected based on a specified operation on dataconveyed with the ray and data stored in the complet. To explore suchflags, it's first helpful to understand the different types of childnodes allowed within a BVH, as well as the various hit types that theTTU 700 can return to the SM. Example node types are:

-   -   A child complet (i.e., an internal node)        By default, the TTU 700 continues traversal by descending into        child complets.    -   A triangle range, corresponding to a contiguous set of triangles        within a triangle buffer    -   (1) By default, triangle ranges encountered by a ray are handled        natively by the TTU 700 by testing the triangles for        intersection and shortening the ray accordingly. If traversal        completes and a triangle was hit, default behavior is for the        triangle ID to be returned to SM 1840, along with the t-value        and barycentric coordinates of the intersection. This is the        “Triangle” hit type.    -   (2) By default, intersected triangles with the alpha bit set are        returned to SM 1840 even if traversal has not completed. The        returned traversal stack contains the state required to continue        traversal if software determines that the triangle was in fact        transparent.    -   (3) Triangle intersection in some embodiments is not supported        for beams, so encountered triangle ranges are by default        returned to SM 1840 as a “TriRange” hit type, which includes a        pointer to the first triangle block overlapping the range,        parameters specifying the range, and the t-value of the        intersection with the leaf bounding box.    -   An item range, consisting of an index (derived from a        user-provided “item range base” stored in the complet) and a        count of items.

By default, item ranges are returned to SM 1840 as an “ItemRange” hittype, consisting of for example an index, a count, and the t-value ofthe intersection with the leaf bounding box.

-   -   An instance node.

The TTU 700 in some embodiments can handle one level of instancingnatively by transforming the ray into the coordinate system of theinstance BVH. Additional levels of instancing (or every other level ofinstancing, depending on strategy) may be handled in software. The“InstanceNode” hit type is provided for this purpose, consisting of apointer to the instance node and the tvalue of the intersection with theleaf bounding box. In other implementations, the hardware can handletwo, three or more levels of instancing.

In addition to the node-specific hit types, a generic “NodeRef” hit typeis provided that consists of a pointer to the parent complet itself, aswell as an ID indicating which child was intersected and the t-value ofthe intersection with the bounding box of that child.

An “Error” hit type may be provided for cases where the query or BVH wasimproperly formed or if traversal encountered issues during traversal.

A “None” hit type may be provided for the case where the ray or beammisses all geometry in the scene.

How the TTU handles each of the four possible node types is determinedby a set of node-specific mode flags set as part of the query for agiven ray. The “default” behavior mentioned above corresponds to thecase where the mode flags are set to all zeroes.

Alternative values for the flags allow for culling all nodes of a giventype, returning nodes of a given type to SM as a NodeRef hit type, orreturning triangle ranges or instance nodes to SM using theircorresponding hit types, rather than processing them natively within theTTU 700.

Additional mode flags may be provided for control handling of alphatriangles.

Exemplary Computing System

Systems with multiple GPUs and CPUs are used in a variety of industriesas developers expose and leverage more parallelism in applications suchas artificial intelligence computing. High-performance GPU-acceleratedsystems with tens to many thousands of compute nodes are deployed indata centers, research facilities, and supercomputers to solve everlarger problems. As the number of processing devices within thehigh-performance systems increases, the communication and data transfermechanisms need to scale to support the increased data transmissionbetween the processing devices.

FIG. 21 is a conceptual diagram of a processing system 1900 implementedusing the PPU 1700 of FIG. 15, in accordance with an embodiment. Theexemplary system 1900 may be configured to implement one or more methodsdisclosed in this application. The processing system 1900 includes a CPU1930, switch 1912, and multiple PPUs 1700 each and respective memories1704. The NVLink 1710 provides high-speed communication links betweeneach of the PPUs 1700. Although a particular number of NVLink 1710 andinterconnect 1702 connections are illustrated in FIG. 21, the number ofconnections to each PPU 1700 and the CPU 1930 may vary. The switch 1912interfaces between the interconnect 1702 and the CPU 1930. The PPUs1700, memories 1704, and NVLinks 1710 may be situated on a singlesemiconductor platform to form a parallel processing module 1925. In anembodiment, the switch 1912 supports two or more protocols to interfacebetween various different connections and/or links.

In another embodiment (not shown), the NVLink 1710 provides one or morehigh-speed communication links between each of the PPUs 1700 and the CPU1930 and the switch 1912 interfaces between the interconnect 1702 andeach of the PPUs 1700. The PPUs 1700, memories 1704, and interconnect1702 may be situated on a single semiconductor platform to form aparallel processing module 1925. In yet another embodiment (not shown),the interconnect 1702 provides one or more communication links betweeneach of the PPUs 1700 and the CPU 1930 and the switch 1912 interfacesbetween each of the PPUs 1700 using the NVLink 1710 to provide one ormore high-speed communication links between the PPUs 1700. In anotherembodiment (not shown), the NVLink 1710 provides one or more high-speedcommunication links between the PPUs 1700 and the CPU 1930 through theswitch 1912. In yet another embodiment (not shown), the interconnect1702 provides one or more communication links between each of the PPUs1700 directly. One or more of the NVLink 1710 high-speed communicationlinks may be implemented as a physical NVLink interconnect or either anon-chip or on-die interconnect using the same protocol as the NVLink1710.

In the context of the present description, a single semiconductorplatform may refer to a sole unitary semiconductor-based integratedcircuit fabricated on a die or chip. It should be noted that the termsingle semiconductor platform may also refer to multi-chip modules withincreased connectivity which simulate on-chip operation and makesubstantial improvements over utilizing a conventional busimplementation. Of course, the various circuits or devices may also besituated separately or in various combinations of semiconductorplatforms per the desires of the user. Alternately, the parallelprocessing module 1925 may be implemented as a circuit board substrateand each of the PPUs 1700 and/or memories 1704 may be packaged devices.In an embodiment, the CPU 1930, switch 1912, and the parallel processingmodule 1925 are situated on a single semiconductor platform.

In an embodiment, the signaling rate of each NVLink 1710 is 20 to 25Gigabits/second and each PPU 1700 includes six NVLink 1710 interfaces(as shown in FIG. 21, five NVLink 1710 interfaces are included for eachPPU 1700). Each NVLink 1710 provides a data transfer rate of 25Gigabytes/second in each direction, with six links providing 1700Gigabytes/second. The NVLinks 1710 can be used exclusively forPPU-to-PPU communication as shown in FIG. 21, or some combination ofPPU-to-PPU and PPU-to-CPU, when the CPU 1930 also includes one or moreNVLink 1710 interfaces.

In an embodiment, the NVLink 1710 allows direct load/store/atomic accessfrom the CPU 1930 to each PPU's 1700 memory 1704. In an embodiment, theNVLink 1710 supports coherency operations, allowing data read from thememories 1704 to be stored in the cache hierarchy of the CPU 1930,reducing cache access latency for the CPU 1930. In an embodiment, theNVLink 1710 includes support for Address Translation Services (ATS),allowing the PPU 1700 to directly access page tables within the CPU1930. One or more of the NVLinks 1710 may also be configured to operatein a low-power mode.

FIG. 22 illustrates an exemplary system 1965 in which the variousarchitecture and/or functionality of the various previous embodimentsmay be implemented. The exemplary system 1965 may be configured toimplement one or more methods disclosed in this application.

As shown, a system 1965 is provided including at least one centralprocessing unit 1930 that is connected to a communication bus 1975. Thecommunication bus 1975 may be implemented using any suitable protocol,such as PCI (Peripheral Component Interconnect), PCI-Express, AGP(Accelerated Graphics Port), HyperTransport, or any other bus orpoint-to-point communication protocol(s). The system 1965 also includesa main memory 1940. Control logic (software) and data are stored in themain memory 1940 which may take the form of random access memory (RAM).

The system 1965 also includes input devices 1960, the parallelprocessing system 1925, and display devices 1945, i.e. a conventionalCRT (cathode ray tube), LCD (liquid crystal display), LED (lightemitting diode), plasma display or the like. User input may be receivedfrom the input devices 1960, e.g., keyboard, mouse, touchpad,microphone, and the like. Each of the foregoing modules and/or devicesmay even be situated on a single semiconductor platform to form thesystem 1965. Alternately, the various modules may also be situatedseparately or in various combinations of semiconductor platforms per thedesires of the user.

Further, the system 1965 may be coupled to a network (e.g., atelecommunications network, local area network (LAN), wireless network,wide area network (WAN) such as the Internet, peer-to-peer network,cable network, or the like) through a network interface 1935 forcommunication purposes.

The system 1965 may also include a secondary storage (not shown). Thesecondary storage includes, for example, a hard disk drive and/or aremovable storage drive, representing a floppy disk drive, a magnetictape drive, a compact disk drive, digital versatile disk (DVD) drive,recording device, universal serial bus (USB) flash memory. The removablestorage drive reads from and/or writes to a removable storage unit in awell-known manner.

Computer programs, or computer control logic algorithms, may be storedin the main memory 1940 and/or the secondary storage. Such computerprograms, when executed, enable the system 1965 to perform variousfunctions. The memory 1940, the storage, and/or any other storage arepossible examples of computer-readable media.

The architecture and/or functionality of the various previous figuresmay be implemented in the context of a general computer system, acircuit board system, a game console system dedicated for entertainmentpurposes, an application-specific system, and/or any other desiredsystem. For example, the system 1965 may take the form of a desktopcomputer, a laptop computer, a tablet computer, servers, supercomputers,a smart-phone (e.g., a wireless, hand-held device), personal digitalassistant (PDA), a digital camera, a vehicle, a head mounted display, ahand-held electronic device, a mobile phone device, a television,workstation, game consoles, embedded system, and/or any other type oflogic.

Machine Learning

Deep neural networks (DNNs) developed on processors, such as the PPU1700 have been used for diverse use cases, from self-driving cars tofaster drug development, from automatic image captioning in online imagedatabases to smart real-time language translation in video chatapplications. Deep learning is a technique that models the neurallearning process of the human brain, continually learning, continuallygetting smarter, and delivering more accurate results more quickly overtime. A child is initially taught by an adult to correctly identify andclassify various shapes, eventually being able to identify shapeswithout any coaching. Similarly, a deep learning or neural learningsystem needs to be trained in object recognition and classification forit get smarter and more efficient at identifying basic objects, occludedobjects, etc., while also assigning context to objects.

At the simplest level, neurons in the human brain look at various inputsthat are received, importance levels are assigned to each of theseinputs, and output is passed on to other neurons to act upon. Anartificial neuron or perceptron is the most basic model of a neuralnetwork. In one example, a perceptron may receive one or more inputsthat represent various features of an object that the perceptron isbeing trained to recognize and classify, and each of these features isassigned a certain weight based on the importance of that feature indefining the shape of an object.

A deep neural network (DNN) model includes multiple layers of manyconnected perceptrons (e.g., nodes) that can be trained with enormousamounts of input data to quickly solve complex problems with highaccuracy. In one example, a first layer of the DLL model breaks down aninput image of an automobile into various sections and looks for basicpatterns such as lines and angles. The second layer assembles the linesto look for higher level patterns such as wheels, windshields, andmirrors. The next layer identifies the type of vehicle, and the finalfew layers generate a label for the input image, identifying the modelof a specific automobile brand.

Once the DNN is trained, the DNN can be deployed and used to identifyand classify objects or patterns in a process known as inference.Examples of inference (the process through which a DNN extracts usefulinformation from a given input) include identifying handwritten numberson checks deposited into ATM machines, identifying images of friends inphotos, delivering movie recommendations to over fifty million users,identifying and classifying different types of automobiles, pedestrians,and road hazards in driverless cars, or translating human speech inreal-time.

During training, data flows through the DNN in a forward propagationphase until a prediction is produced that indicates a labelcorresponding to the input. If the neural network does not correctlylabel the input, then errors between the correct label and the predictedlabel are analyzed, and the weights are adjusted for each feature duringa backward propagation phase until the DNN correctly labels the inputand other inputs in a training dataset. Training complex neural networksrequires massive amounts of parallel computing performance, includingfloating-point multiplications and additions that are supported by thePPU 1700. Inferencing is less compute-intensive than training, being alatency-sensitive process where a trained neural network is applied tonew inputs it has not seen before to classify images, translate speech,and generally infer new information.

Neural networks rely heavily on matrix math operations, and complexmulti-layered networks require tremendous amounts of floating-pointperformance and bandwidth for both efficiency and speed. With thousandsof processing cores, optimized for matrix math operations, anddelivering tens to hundreds of TFLOPS of performance, the PPU 1700 is acomputing platform capable of delivering performance required for deepneural network-based artificial intelligence and machine learningapplications.

All patents & publications cited above are incorporated by reference asif expressly set forth.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

The invention claimed is:
 1. An interruptible processing systemproviding a forward progress guarantee comprising: a storage that storesa representation of a three dimensional (3D) scene; at least one stack;a traversal circuitry operatively coupled to the storage and the atleast one stack, the traversal circuitry configured to receive, from oneor more processors, a plurality of queries including ray information fora plurality of rays, process the plurality of rays in parallel, againstthe representation of the 3D scene to determine bounding volumes orgeometric primitives in the 3D scene the plurality of rays intersect,and store information about bounding volumes or geometric primitives thetraversal circuit determines are intersected by the ray in the at leastone stack; and preemption circuitry operatively coupled to the traversalcircuitry and configured to ensure the traversal circuitry makes forwardprogress in processing each of the plurality of rays being processed inparallel against the 3D scene before stopping further forward progressof the traversal circuitry in response to assertion of a preemptionsignal.
 2. A ray tracing device comprising: a storage that stores arepresentation of a three dimensional (3D) scene; a traversal circuitoperatively coupled to the storage, the traversal circuit configured toreceive, from one or more processors, information about at least one rayand process the at least one ray against the representation of the 3Dscene; and a preemption circuit operatively coupled to the traversalcircuit and configured to, in response to receiving a preemption signal,ensure the traversal circuit makes forward progress in processing the atleast one ray against the 3D scene and other rays being processed inparallel with at least one ray by the traversal circuit at the time thepreemption signal is received before preempting the traversal circuit.3. The ray tracing device of claim 2, further including at least onestack connected to the traversal circuit, and wherein the preemptioncircuit includes a stack manager operatively coupled to the at least onestack, the stack manager being configured to ensure the traversalcircuit modifies the at least one stack before preempting the traversalcircuit.
 4. The ray tracing device of claim 3, wherein modifying the atleast one stack includes storing information about a bounding volume ora geometric primitive in the 3D scene that the traversal circuitdetermines is intersected by the at least one ray.
 5. The ray tracingdevice of claim 3, wherein modifying the at least one stack includespopping an entry from the at least one stack.
 6. The ray tracing deviceof claim 3, wherein the traversal circuit is further configured toreceive, from a processor, a query including ray information for the atleast one ray, traverse the 3D scene to determine bounding volumes orgeometric primitives in the 3D scene the at least one ray intersects inresponse to the query, and store information about bounding volumes orgeometric primitives the traversal circuit determines are intersected bythe ray in the at least one stack.
 7. The ray tracing device of claim 3,wherein the traversal circuit is further configured to receive, from aprocessor, a query including ray information for the at least one ray,traverse the 3D scene to determine bounding volumes or geometricprimitives in the 3D scene the at least one ray intersects in responseto the query, and store information about results of the traversal inthe at least one stack, wherein the traversal circuit is preemptedbefore completion of the query and information stored in the at leastone stack when the traversal circuit is preempted is transmitted to theprocessor.
 8. The ray tracing device of claim 2, wherein the traversalcircuit is configured to process a plurality of rays against therepresentation of the 3D scene, and the preemption circuit is configuredto ensure the traversal circuit makes forward progress in processingeach of the plurality of rays against the 3D scene before preempting thetraversal circuit.
 9. The ray tracing device of claim 2, wherein thepreemption circuit uses an inference that forward progress has occurred.10. The ray tracing device of claim 2, wherein the preemption signal isreceived from the one or more processors during the processing of therays against the representation of the 3D scene by the traversalcircuit.
 11. A method implemented by circuitry comprising a traversalcircuit coupled to a storage that stores a representation of a threedimensional (3D) scene and a preemption circuit operatively coupled tothe traversal circuit, the method comprising: receiving informationabout at least one ray from one or more processors; processing, via thetraversal circuit, e at least one ray against the representation of the3D scene; and in response to receiving a preemption signal, ensuring,via the preemption circuit, that the traversal circuit makes forwardprogress in processing the at least one ray against the 3D scene andother rays being processed in parallel with the at least one ray by thetraversal circuit at the time the preemption signal is received beforepreempting the traversal circuit.
 12. The method of claim 11, whereinensuring that the traversal circuit makes forward progress includesensuring that the traversal circuit modifies at least one stackconnected to the traversal circuit before preempting the traversalcircuit.
 13. The method of claim 12, wherein modifying the at least onestack includes storing information about a bounding volume or ageometric primitive in the 3D scene that the traversal circuitdetermines is intersected by the at least one ray.
 14. The method ofclaim 12, wherein modifying the at least one stack includes popping anentry from the at least one stack.
 15. The method of claim 12, furthercomprising: receiving, via the traversal circuit, a query including rayinformation for the at least one ray, traversing the 3D scene todetermine bounding volumes or geometric primitives in the 3D scene theat least one ray intersects in response to the query, and storinginformation about bounding volumes or geometric primitives the traversalcircuit determines are intersected by the ray in the at least one stack.16. The method of claim 12, further comprising: receiving, via thetraversal circuit, a query including ray information for the at leastone ray from a processor, traversing the 3D scene to determine boundingvolumes or geometric primitives in the 3D scene the at least one rayintersects in response to the query, and storing information aboutresults of the traversal in the at least one stack, wherein thetraversal circuit is preempted before completion of the query andinformation stored in the at least one stack when the traversal circuitis preempted is transmitted to the processor.
 17. The method of claim11, further comprising: processing, via the traversal circuit, aplurality of rays against the representation of the 3D scene, andensuring, via the preemption circuit, that the traversal circuit makesforward progress in processing each of the plurality of rays against the3D scene before preempting the traversal circuit.
 18. The method ofclaim 11, wherein the preemption signal is received from the one or moreprocessors during the processing of the rays against the representationof the 3D scene by the traversal circuit.
 19. A ray tracing devicecomprising: a storage that stores a representation of a threedimensional (3D) scene; a traversal circuit operatively coupled to thestorage, the traversal circuit configured to simultaneously process aplurality of rays against the representation of the 3D scene; and apreemption circuit operatively coupled to the traversal circuit andconfigured to ensure the traversal circuit makes forward progress inprocessing at least one ray of the plurality of rays against the 3Dscene before preempting the traversal circuit and stop the traversalcircuit from processing only a subset of the plurality of rays whenpreempting the traversal circuit.
 20. A method implemented by circuitrycomprising a traversal circuit coupled to a storage that stores arepresentation of a three dimensional (3D) scene and a preemptioncircuit operatively coupled to the traversal circuit, the methodcomprising: simultaneously processing, via the traversal circuit, aplurality of rays against the representation of the 3D scene; ensuring,via the preemption circuit, that the traversal circuit makes forwardprogress in processing at least one ray of the plurality of rays againstthe 3D scene before preempting the traversal circuit; and stopping, viathe preemption circuit, the traversal circuit from processing only asubset of the plurality of rays when preempting the traversal circuit.